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Providing no-js fallbacks for clientside MV* frameworks

I just recently got introduced to MV* frameworks and have taken a chance to try out Ember.js with the TodoMVC app tutorial they have on their site.
I was considering using Ember for one of my upcoming projects (a Ruby on Rails CRUD app, similar to Twitter in some of the functionality), but I'm still a bit confused and before I take a final decision I would love it if somebody could clear the following concerns:
Is it a good idea to use such an advanced framework as Ember for a medium-sized multi-page CRUD app? Will it improve development time and maintenance compared to an interactivity layer built with jQuery's DOM manipulation and AJAX capabilities? Or is using Ember (and the like) only good when developing complex single-page apps (e.g.: Grooveshark)?
Considering the app will be developed using Rails, and assuming Ember will be used, is it going to be possible to offer a fallback with basic functionality for browsers with JavaScript disabled and/or for search engine crawlers? Will it require code duplication or other dirty tricks? Do you know of any technique that can be used to achieve it?
Will it be possible to adapt the website for mobile browsing (using only CSS) with valid results, or will the overhead imposed by running Ember on the phone make it hard for the device to render the website in a way that keeps it responsive?

We're in the middle of a pretty big Ember project right now, so here are my thoughts on your questions.
We've found Ember to be really productive for creating rich UIs for our single page app, but I don't know that it's going to be that much more helpful if you're creating an app designed for traditional multi-page (viewing pages, submitting forms, etc) layout.
I think this is the clincher - Ember is completely JS-based, so if you need to support browsers without JS, you'd basically have to write a parallel application. If this is a hard requirement for your app, I think Ember (or any MV* JS framework) would be out of the question
We've had very few performance issues on mobile - our site is fully responsive and renders on everything from Blackberries to the latest Chrome on desktop with good performance.

#Scott Rankin, has addressed most of the concerns with going with the Pure Ember approach. I'll add one quick way to make this decision.
Go with Ember/MVVM if the application is behind a login. Then you don't have to consider search engines, as the content is generally private and not supposed to be indexed.
For SEO you have to build atleast part of your content such that it is indexable. A good example of this is the Discourse application. They use Ember but also send down some generated html along with the app html slugs, so that search engines can index them. You can read about their approach here.

We have a different approach which can be seen as a fall back: We pre-render a static version of each page in the application (daily scheduled task). This static version is stored on the server as HTML file. Whenever we sniff as spider/ robot user agent, we deliver that version.

JavaScript GUI library which is usable from ClojureScript?

On one of my ongoing web development projects, we need to create a page (really a mini-application of its own) which will have an interactive, "Web 2.0" interface, more like a graphical desktop application than a typical web page.
I'm hoping to use a higher-level language which can compile down to JS for the client-side logic, rather than programming directly in JS, and ClojureScript is my first choice.
I'm looking for a JS GUI library which can be used from within ClojureScript, and which supports a style of programming similar to "desktop" GUI libraries like Swing, QT, wxWidgets, and so on. It should allow me to create popup <div> "windows", put widgets such as buttons/labels/etc. in them, attach event handlers to the widgets, and so on. Do you know of one which would work?

Clojurescript is still young - I don't think there are mature frameworks of that caliber (yet?). Having said that, I'm building a UI using the following components:
Twitter Bootstrap + JQuery for visuals and UI elements
Chas Emerick's Friend for server-side authentication
Compojure + Ring + Jetty for the server
Crate (Clojurescript Hiccup) for html/dom generating
Jayq for JQuery wrapping
Fetch and/or Shoreleave's remotes for client-server RPC
Shoreleave's pubsub mechanism or Clojure's add-watch for data binding
A good feature of Clojure and Clojurescript are that they favour simple functions on generic data structures. That's why you tend to see several smaller libraries that do one specific job, but work really well together, as opposed to giant monolithic frameworks with an object model that requires you to adopt one way of thinking completely.

You should consider Google Closure Library. It has all features you ask and is definitely usable from ClojuresScript, I've played around with it. See http://closure-library.googlecode.com/svn-history/r1430/docs/index.html

I think ExtJS would fit the best in your case, even though this is pure JS. This is the most advanced web GUI library at the moment and it makes possible to create web applications the way similar the one using the desktop languages and IDEs. Just check out their demos. I don't think there will be something similar for the ClojureScript in the foreseen future.

Need help determining how to approach building my project

I'd like to create something similar to a family tree online app (like geni.com). I'm unsure what languages I should use to build it with. My IT strong points aren't in programming and this project is going to require me to sit down and learn some languages. My problem right now is that I don't know what languages I should use.
So with the idea of a family tree online app in mind here are some of the specifications.
- I do not want to use flash.
- The app needs to be zoomable and scrollable (sort of like google maps)
- The app needs to be able to add content without reloading the page. Perhaps there's a little "+" sign and when I click it, I can add a tag/title/description/picture
- The app needs to be able to save your work for that user to retrieve later on.
- The layout that a user is able to create in is sort of widget based where the user can add a new bubble and then in that bubble they are able to add text or content.
I started programming this with HTML5 canvas and Javascript, but I'm stuck on creating a connection to the database that isn't directly from Javascript (because that seems very insecure to me). But I'm not just stumped on how to interact securely with the database (I don't even have a database picked out), but also I'm concerned that I won't be able to build out the app with just javascript and may need something else like ajax or something but I'm unfamiliar with what each language does nowadays.

If you are starting from scratch, then the best language to use is the one you are most comfortable with. Alternately, if you don't plan to be developing the whole thing yourself and you already have some other interested parties on board then the best language to use is the one that the majority of you are comfortable with. If it's just you and you do not yet have any favorites, then look around and play with a few - it's the only way to find out if you will actually like / be effective with them.
That being said, a few of the more likely candidates these days are:
JavaScript: Long gone are the days when this language was simply a way to put the D in DHTML. These days JavaScript is a viable client and server-side language. (Others here have already recommended Node.js -- I'd also recommend NPM (node package manager) to handle your dependencies). With a little bit of planning you can reuse most of your application code on both the client and the server side. On the downside, most of the server side technology is very new (only a few years old at most) and so you may find yourself implementing tools for use in your application rather than your application itself. Finding servers that support it will also be harder, again on account of it's age.
Perl: At the opposite end of the spectrum of age, we find Perl - the first commonly deployed language used to make web applications it still powers a great variety of useful websites out there (include new ones such as Pinboard.) The tools that are popular on CPAN have been vetted under fire. The good news is that it is not going anywhere anytime soon. The bad news is, you might have to search a little harder to find a module that supports [that newest, baddest thing that just came out yesterday].
PHP: The BASIC (or Perl, depending on who you ask) of the modern web, PHP was designed from the ground up to do one thing - make building dynamic web pages easier. Its popularity means that there is quite a lot of server support (PHP + Apache + MySQL is the Model T Ford of web servers -- everyone can afford one) and an enormous amount of pre-built code available for perusal. However, like BASIC, PHP's strength is also its greatest weakness. Almost anyone can write something that works in PHP ... how well it works depends on who wrote it. The caveat emptor that applies to all code snippets found on the web applies in spades to snippets of code written in PHP.
Python: The language that made programing fun again (at least for those who can see past the significant whitespace and lack of blocks / anonymous functions and overlook < 3.x's issues with non-ASCII out of the box.) It's a general-purpose, flexible and multi-paradigm language with quite a substantial standard library (but without .NET or Java's incredible bloat). In addition, quite a large amount of work has been done in it, so there is a good chance that what you need has been already developed by somebody else. Plus, it can make you fly.
Ruby (with or without Rails): The language that made the web fun, coupled, if you so desire, with the framework that made MVC cool. There is lots of documentation out there, and a great community, with many prebuilt tools (called gems) from which to pick and choose - free and cheap servers are not as common as their PHP counterparts, but they are likely to be of higher quality (when chosen at random).
All that being said, they are all great languages for web development. What matters is not what we think you should use ... but what you are most likely to be effective with. All of the languages listed above are mainstream (or will be in the next two years), easy-to-learn and easy-to-write languages. You cannot go wrong, no matter what you choose to start off with.
Alternately, if you want something a little more difficult, or less mainstream ... I am working with .NET applications at work, and with Lisp (SBCL)-based services in my spare time. I have heard great things about Lua and Java too ... there are at least two C++ web frameworks out there ... and I'm sure that there is somebody is having fun building a web service in COBOL with a FORTRAN backend. ;-)

As someone has already pointed out, you will need to work with a server side language as well. (Ruby, Python, PHP) You are exactly right there you should not be attempting a database connection via javascript in the browser.
You'll need to build out a server side application to handle the basic operations of your application.
I'd strongly recommend reading up on the MVC design pattern, and possibly looking into Ruby on Rails as your backend framework, it plays very nicely with ajax like features, and has a somewhat shorter learning curve, I believe, than some other frameworks / languages.

You will need server side scripts in a language like PHP or Ruby on Rails to interact with a database.

If you're already familiar with HTML5 and JavaScript, may I recommend using Node.JS? It's about the closest you'll get to what you already know with browser development. It can also hook in with database systems which are closer to the HTML5-suggested IndexedDB.
with that in mind...
If you're inexperienced with programming and programming languages, then the app you describe will involve a pretty big learning curve. While Flash and Flex have really nice interfaces to build apps with click-and-drag, the tools for HTML5 are much less mature.
That's not saying it's not possible with HTML5. Just that there's still some time to wait before people create tools to bring the app building process closer to what Adobe provides.

Why should I use Flex?

In a recent conversation, I mentioned that I was using JavaScript for a web application. That comment prompted a response: "You should use Flex instead. It will cut your development time down and JavaScript is too hard to debug and maintain. You need to use the right tool for the right job." Now, I don't know too much about Flex, but I personally don't feel like JavaScript is too hard to debug or maintain, especially if you use a framework. JavaScript is also one of the most used languages right now, so it would seem a better choice in that regard too. However, his reply piqued my interest. Would Flex be a good choice for a distributable web app for which 3rd party developers could build add-ons? What are the advantages of using it vs. a JavaScript framework? What are some of the disadvantages?

I have recently started to develop Flex applications, and I personally find it a refreshing framework for the web.
You get a state-ful application that runs completely client side. You get no worries about cross-browser portability that you do with JavaScript, and you get some really neat things such as effects, graphing, and rich user interface components.
Flex also makes it easy to communicate to webservices and the XML parsing via ECMA is insanely powerful and simple.
I'm glad I have made the switch. As far as how popular it is...I'm not really sure, but I am fairly certain that the developer base is expanding rapidly.
The only real disadvantage I can think of is a flash player requirement, but I would say it is pretty safe to assume that most browser support flash player; even konquerer in Linux is supported; much more so then a silverlight runtime (which I NEVER plan on installing)

Here is my experience: you really need to consider 2 things separately - development and the end-user experience. Flex shines in the first area:
ActionScript is a nice mixture of Java and JavaScript so you get a familiar language with strong support for OOP
debugging is far easier than what you can achieve in JavaScript
Flex framework is component-oriented and event-driven which helps in creating rich user interfaces (HTML was not really created to support application UI scenarios)
On the other hand, the end-user experience is worse when running a Flex app compared to an AJAX app. First, you need to have Flash Player installed but this is probably not an issue for most computers today. Bigger problems are with usability - Flash Player handles all UI interactions (instead of a browser) so the password manager doesn't work, text fields don't remember previous entries, Ctrl+T and middle-clicking doesn't work, text search doesn't work etc. etc.
My advice would be - if you are developing an application (rich UI, relatively separated from the rest of the web), go for Flex as it will save you time, money and will make your users happier by providing richer functionality and shorter periods between new versions. On the other hand, if your application needs to be tightly integrated with the web and you want your users to be able to use features of their browsers, go with AJAX.
Nice example is Google Docs vs Buzzword. Buzzword is much more feature rich (for instance, text can flow around an image from both sides which is something you could never ever achieve in DHTML) but Google still decided to go for an AJAX version because they are the "web company". There is no right or wrong in doing it the one or the other way, it's just different and it's important to consider who your end users are.

I would push you towards standard web development technologies in most cases. Javascript is no longer a great challenge to debug or maintain with good libs like jQuery/Prototype to iron out some of the browser inconsistencies and tools like Firebug and the MS script debugger to help with debugging.
There are cases when Flash is a better option, but only in cases where you are doing complex animations. And, if you are willing to invest the effort, most animations can be achieved without resorting to flash. A couple of examples...
Flash content is not as accessible as other content.
This will not only affect people with out flash, but also search engine spiders. There may be some hacks to help get around this now, but I think that most flash content will never be indexed by google.
Flash breaks the web UI.
For example:
If I click my mouse wheel on a link,
that link is opened in a background
tab. In a flash app there is no way
to simulate this behavior.
If I select text in my browser and
right-click I get options provided
by the browser that include things
like "Search Google for this text".
In a flash app those options are no
longer there.
If I right click on a link or an
image I get a different set of
options that are not available in a
flash app. This can be very
frustrating to a user who is not
"flash savvy".

GWT lets you do the same stuff as Flex for the most part, and handles all the browser compatibility issues, AND lets you code/debug in Java with your favorite IDE.
All without having to learn a new language (or pay Adobe $$$ for the flex IDE you'll need to do anything real).
Flex has some prettier UI widgets than GWT has out of the box, but there's a ton of 3rd party widgets (such as GWT-EXT-JS) you can use - or, you can use your existing favorite JS widgets with GWT.
Check it out if you haven't: http://code.google.com/webtoolkit/

I can't be sure if it was myself, or someone else who made that statement but I would definitely be one to say 'use the right tool for the job'.
Flex has a large community behind it, and is well hyped by Adobe's platform evangelism team. Now, as far as replacing JavaScript, that sounds like a very broad spectrum discussion point. Flex is not a replacement for JavaScript. What it does, it does well, however. That is, 3D, drawing, and data rendering whether in chart or table form. Flex also has the power of ActionScript 3 behind it which allows you to do much of what Flash does in cooperation with the MXML frontend components without ever touching the timeline or keyframes.
In a way, Flex is the .NET of Flash and Rich Internet Application development. It uses the same datasource concepts, and component focused design structures which make it easy, and fast to develop in.
The real question is, what are you trying to achieve? What is the end goal?
As to the debugging point, Flex has a true debugger and profiler within the Flex Builder IDE. JavaScript, unfortunately, has different syntax and execution between browsers due to the nature of JavaScript engines in modern browsers. Flex, because it is essentially Flash, uses the same rendering engine in all browsers due to the use of the Flash plugin.
Hope that clears a few things up. :)

Flex has a lot of extra overhead:
New language
Clients must have flash installed (might need to install, might not be able to)
Clients must download flex framework (few hundred kilobytes)
Flex content is not indexed by search engines (contrary to what Google might claim)
Flex has one main advantage:
- Better at building rich interfaces (see Picnik.com, etc)
For example, in Flex, it is easy to create a custom styled dialog box, complete with drop shadows, inner glows, animated open, whatever you might want.
In summary, use Flex if you need the extra richness.

Aside from what's already been mentioned here, another major difference is that JavaScript is dynamically typed and ActionScript is statically typed. Whether that's good or bad will depend on your point of view :).

If you want your web application to look like it's not a web application, Flex is pretty good. You also get to sidestep all the messiness of making HTML+JS look like a real app. For something which is essentially a website, Flex might not be the best choice, but if you really want to write an application which happens to be accessed through the browser, it's quick to develop with and gives great looking results.

You should try Google Gears instead. Create your application, add some Gears to it, and you can greatly increase the speed (and reliability) of your application.
http://gears.google.com/
Essentially Google gears gives you access to two useful things for any application: offline data storage, and native threading control (allowing updates/computations to run in the background and not slow down the users computer).
The really nice thing is, you can use whatever Framework you like for your application, as long as data storage/retrieval and server side communication is handled with JavaScript.
It also allows you to cache whatever files client side you want, which is especially useful when you want to avoid that 'flickering' look in the browser while some needed image is being downloaded by the browser.

A few reasons to consider Flex:
The control library is much richer in Flex than anything you can do with JS/DHTML. The charting controls are killer for business apps and things like the DataGrid / AdvancedDataGrid are pretty well ahead of anything you can do with HTML.
The Flex framework was designed for building applications. It abstracts away the "frame-based" concepts in the Flash Player to really make it easy to build apps. It has a well-designed component hierarchy that makes it easy to extend any of the standard controls. It also has a pretty intuitive event model for handles user inputs and makes it easy to have any of your controls dispatch custom events that can bubble up to parent components or get routed through a central event dispatcher. While it may be possible to do this with JS/DHTML, I don't think it's nearly as easy and it certainly wasn't designed for it.
You can take a Flex application and quickly deploy it to the desktop with the AIR runtime. AIR also offers additional APIs for things like local system access, embedded SQLite DB, etc. Gears offers something similar but it does require a browser. Granted, AIR requires the AIR runtime but at least it's purposed towards building desktop apps.
You can build a very rich, very sexy UI that will knock your users socks' off. As programmers we might not care about UX but our users do. Part of the reason why Apple is having a lot of success lately is because they really value UX and users/consumers are taking note of this.
The biggest con I think is that if you are really used to Java or C#, the ActionScript language will seem a bit limiting. If you're comparing it JavaScript, it's at par or maybe slightly better.
A lot of people will rail on Flash Player (or AIR) because it's not "standard-based." If we were only willing to use sites that were 100% standards compliant and free of plugins, we wouldn't have YouTube today. Or pretty much any other site that does interesting data visualization you can't do with HTML/JS (or at least, not with a sane level of effort). Adobe has been pretty progressive in opening up the Flex framework, Blaze DS (for backend Java development), publishing the AMF spec and starting the Open Screen Alliance to push Flash Player to mobile devices. Flash Player, Flex, Flex Builder and Blaze DS all have public JIRA bug trackers. I'd say there is a good chance that Flash Player itself will be open source within the next 2-3 years. I think Adobe is continuing to move towards being very open and that the criticisms of the platform being "closed" and "proprietary" are becoming less relevant. I think if developers approach Flex/FP with an open mind that they would really be impressed with how it all fits together.

This comparison table was good enough to make me decide what to use . I prefered javascript:)
http://askmeflash.com/article_m.php?p=article&id=11

Should I use Google Web Toolkit for my new webapp?

I would like to create a database backed interactive AJAX webapp which has a custom (specific kind of events, editing) calendaring system. This would involve quite a lot of JavaScript and AJAX, and I thought about Google Web Toolkit for the interface and Ruby on Rails for server side.
Is Google Web Toolkit reliable and good? What hidden risks might be if I choose Google Web Toolkit? Can one easily combine it with Ruby on Rails on server side? Or should I try to use directly a JavaScript library like jQuery?
I have no experience in web development except some HTML, but I am an experienced programmer (c++, java, c#), and I would like to use only free tools for this project.

RoR is actually one of the things the GWT is made to work well with, as long as you're using REST properly. It's in the Google Web Toolkit Applications book, and you can see a demo from the book using this kind of idea here. That's not to say that you won't have any problems, but I think the support is definitely out there for it.
There's a neat project for making RoR/GWT easy that you can find here (MIT license). I haven't had a chance to try it out yet, but it looks like a good amount of thought has been put into it. One catch is that it looks like it hasn't been fully tested with 2.1 Rails yet, just 2.0, so you may run into a few (probably minor and fixable) errors.

If you are looking to integrate GWT with non-Java backends such as ROR, PHP etc., you should bear in mind that GWT 1.5 now supports JavaScript Overlay types. This feature lets you write classes that can be mapped over the top of native JavaScript objects to easily provide accessor methods for properties of those objects and other extended functionality.
See this link for more details:
JavaScript Overlay Types
So you could return JSON encoded data from your backend via AJAX calls, parse it into a JavaScript Object and then access the data through your GWT Java code using the overlay classes you've created. Or when you render your page you can render static config data as JavaScript Objects and read it in via this mechanism, rather than having to do an AJAX call to grab the data.

If you know JAVA, and have somewhere you can host it (like a tomcat or glassfish container) I would recommend that much more than using Ruby for the back end. The main reason is that then you can share all of your objects, and use the built in RPC mechanism. I've done this for quite a lot of our projects and it's a huge timesaver, not to mention that the code is less error prone, because you don't convert your java objects to anything and then back again.
I have linked my GWT with Rails before, using the to_json function in Rails and then reading the JSON in GWT. It's all supported, but it is far more annoying than just doing the back end in JAVA.
Of course if you have cheap hosting, then Java containers are pretty much out of the question, in which case I would think Rails would be the next best thing.

GWT is very high quality with a great community. However you do need to know CSS if you want to adjust the look of things (you will) - CSS can do a lot of the layout, just like regular web if you want it to. Libraries like GWT-ext or ExtGWT can help a bit as they have stunning "out of the box" looks but for a price (extra size to your app).

You can code everything in Java using GWT, and you can integrate existing 3rd party javascript libraries with it. It's very good. I've never used RoR much though, so can't say anything about that.

If you're experienced in Java but not in Javascript/CSS, then GWT is going to be a lifesaver (unless you want to learn them, of course). CSS has so many little fiddly details. It is not uncommon to spend half a day fixing a 2 pixel misalignment that only occurs in IE6.
I am not sure about how easy it would be to use ROR for the back end... It is possible, I am sure, since GWT ajax communication is just servlets. But they provide some really nice functionality for passing Java objects back and forth which you won't be able to utilize if your server isn't also using Java.

I wrote about some of the disadvantages of GWT recently. Mainly, the disadvantages are: long deployment cycle for changes to some parts of the application and a rather steep learning curve. As a seasoned Java programmer, the second should be less of a problem and if you use a seperate backend, the first is also mitigated (as a complete redeploy is primarily required when you change the 'server' part of the application).

GWT is a wonderful framework with lots of potential. Keep in mind that it's still quite new, though. There are some unresolved bugs that can really annoy you, and they usually require ugly workarounds to get past. The community is great but you'll probably end up with a few problems sooner or later that Google can't answer yet.
But hey, I say go for it. The potential for GWT is awesome, and I bet it's future will be bright.

You should definitely use GWT for a new project (it's pretty easy to use in an old project too).
I my experience it's very fast to learn and use. The compiled javascript code is much better than anything you could ever write by hand and it works fast too.
Another benefit is the ability to debug you're code (which is hell with javascript alone)

This blog has inputs from many experienced users of GWT and have some great discussion points. I personally have huge experience with varied UI Frameworks. I will add my two cents. Lets look at fundamental advantages and disadvantages of GWT
Fundamental Advantage
GWT takes the web layer programming to JAVA. So, the obvious advantages of Java start getting into play. It will provide Object Oriented programming. It will also provide great debugging and compile time checks. Since it generates HTML and Javascript, it will also have ability to hide some complexity within its generator.
Fundamental Disadvantage
The disadvantage starts from the same statement. GWT takes the web layer programming to JAVA. If you know JAVA, probably you will never look out for an alternative language to write your business logic. It's self sufficient and great. But when it comes to writing configurations for a JAVA application. We use property files, database, XML etc. We never store configurations in a JAVA class file. Think hard, why is that?
This is because configuration is a static data. It often require hierarchy. It is supposed to be readable. It never requires compilation. It doesn't require knowledge of JAVA programming language. In short, it is a different ball game. Now the question is, how it relates to our discussion?
Now, lets think about a web page. Do you think when we write a web page we write a business logic? Absolutely not. Web page is just a configuration. It is a configuration of hierarchical containers and fields. We need to write business logic for the data that will be captured from and displayed on the web page and not to create the web page itself.
Previous paragraph makes a very very strong statement. This will explain why HTML and XML based web pages are still the most popular ones. XML is the best in business to write configurations. A framework must allow a clear separation of web page from business logic (the goal of MVC framework). By doing this a web designer will be able to apply his skills of visualization and artistry to create brilliant looking web pages just by configuring XMLs and without being bothered about the intricacies of a programming language. Developers will be able to use their best in business JAVA for writing business logic.
Finally, lets talk about the repercussions in direct terms. GWT breaks this principal so it is bound to fail. The cost for developing GWT application will be very high because you will need multiskill programmers to write web pages. The required look and feel will be very hard to achieve. The turn around time of modifying the web page will be very high because of unnecessary compilation. And lastly, since you are writing web pages in JAVA it is very easy to corrupt it with business logic. Unknowingly you will introduce complexities that must be avoided.

You could also consider Grails ("Groovy on Rails") which gives you the benefits of a Rails framework and the use of the Java VM.

Our team recently asked the same question, and we chose to go with GWT, especially since the designer plugin made working with GWT more accessible to non-java experts on the team. Whoever makes this choice, just beware you DON'T use the GWT Designer plugin !! It has not been updated (in at least a year, apparently) to create a GWT application that is compatible with IE8.
Our team had almost completed our application layouts, which were working perfectly in Chrome, FF and Safari. Then they blew up in IE. IE 7 would load partial pages (but not composite includes), and IE8 was not even able to load up the application. It just hung.
The designer plugin has buttons that allow the user to add CellTable widgets that are not IE compatible (CellTable, DeckPanel, Horizontal Panel, Vertical Panel, among others). These will cause intense pain when the layouts have to be re-done in java without assistance from the designer.
Experienced GWT users love it, but the designer plugin will kill you.