Polling is not good enough, and is wasteful.
Something like http://code.google.com/p/gwteventservice/wiki/StartPage would be fine

With huge datasets it is not good to load everything into the jvm at once. We need a container that can load lazily from various sources. I suggest that the container should take in an interface implementation telling it how to get more entries and telling it the size of the whole data set. getObjectById, sorting possibilities, filters etc would also be nice.

Confirm dialog functionaly is needed nearly in every application. Currently everyone needs to implement this by hand. Some of the implementation presentedin this thread: http://vaadin.com/forum/-/message_boards/message/17883
Just cherry pick the best ideas.

The current components are pretty static, once rendered. Animations (in addition to the Notification's fade in/out) would bring some zing to the UI. There are also numerous usability gains in animating with taste.

Currently one can add right click actions to e.g. a tree or a table. The problem with the actions is that they can only be in one level, so an action cannot have sub-actions. This limitation makes it really hard to create user friendly and efficient action menus.

A paging menu would be nice to allow for display of page sets available and number of results required within a table. For example

http://www.sencha.com/examples/explorer.html#localpaging

Editing tables with fields or forms with fields is not enough. I need to link objects between them: automatic drop-downs or radio-buttons for one-to-one or many-to-one, multi-select or checkboxes for one-to-many and many-to-many. Make sure that when read-only a meaningful format is produced.

I would be useful to assign an individual tooltip to each option of an OptionGroup (radio buttons).

Making Vaadin bookmarkable is laborous. There should be some way to make it easier and more systematic to create bookmarks and make the Application able to navigate to the state.

The ThreadLocal is used in several applications to gain static access methods to Application instance. This should be part of the core framework.

Buttons can look like links and it's great.
I sometimes need links that look like Buttons (and I don't want to go to photoshop to create bitmaps ;-) or to digg my css.

myLink.setStyleName("button");

This would apply use the same layout as the current theme when creating a button.

It would be nice if Vaadin had the ability to justify top level menu items in a menubar. For example, we have a requirement that the "Administration" menu item be right justified to keep it separate from other menu items. Other menu items such as File, View etc., are default left justified.

create an an IntelliJ plugin that Offers equivalent functionality to the eclipse plugin.

We are also planning to split "subwindows" and "toplevel" windows, which are currently the same thing on server side although they really are very different beasts.

The distinction between sending a property or a variable from the server-side component over to the client-side widget is too small (none, actually) to be really useful for the programmer.

I wrote lengthier about this at http://wolfie.github.com/2009/12/06/now-i-know.html

Vaddin looks great, but we couldn't use it in our new project because of its performance in terms of bandwidth and speed. Our project generates about 100 TB of bandwidth per month of 'pure' data excluding overhead of UI generation and management. Being bound to contact the server for every button click is unbearable. Why don't you offer an equivalent model that is client-driven? I guess you will find many who may participate in that project. I am in included.

In many situations, I don't like to create a style in the separate CSS file, and to reference it in my code with .addStyleName("myStyle").
But I can live with it.
Except when I write a reusable add-on. I don't wanna create client-side widgets just to have a CssLayout resulting in <div ... style="anything I want">. And I don't want to require the users of the add-on to modify their CSS because of me.
It would be simple to add a method Component.setInnerStyle(String), and I'm sure a lot of Vaadin users would use it.

Expandable table rows would be quite useful to bind extra data on top of the standard row.

http://www.sencha.com/playpen/ext-2.0/examples/grid/grid3.html