Why is there no rock solid framework for RIA (= JS desktop like apps in the browser)?
Here is my list of serious RIA frameworks.
Ext JS
Yahoo YUI
Cappuccino
Qooxdoo
DOJO
Moo Tools
JqueryUI
KendoUI
zeptojs
ExtJS is commercial, Cappuccino uses a custom language, YUI is old, jQueryUI and dojo don't have good tables & panels. Qooxdoo is very nice, but has not managed to build a rich community for whatever reason. I didn't include Angular and Backbone, because they quite different goals it seems to me.
How does Google's polymer/XBL2/Shawdow-Dom fit into the picture? Any emerging libs already building on this?
I work on YUI, and there's definitely lots of active development still going on with it. There's sometimes a bit of confusion with YUI2 and YUI3, with YUI2 being much older and more verbose.
YUI3 is a lot more developer-friendly, and we've worked to build more modern components such as Pure CSS [1] and the YUI App Framework [2]. SmugMug's new site that they released a few days ago (http://www.smugmug.com/) is built on top of YUI, and it looks absolutely fantastic.
If there's anything I can help you out with, definitely feel free to let me know! We've definitely come a long way since YUI2.
[1]: http://purecss.io/
[2]: http://yuilibrary.com/yui/docs/app/