The first thing I've found is that this beta version doesn't support the upgrading of previous version's databases, which is a bit of a bummer for testing purposes really as it means creating a whole new catalog and I'm unlikely to properly put it through its paces if I can't work on an existing catalog.
The dual monitor action is really cool and super useful. You can have your main window showing a grid view for example, and the secondary monitor showing the Loupe or Survey views - very handy. Fullscreen, 1:1 & 3:1 views are available at one click of the mouse.
The organisation of the Library module has been much improved. There is now a "Smart Collection" feature which automatically updates collections with matching criteria such as star-ratings, keywords or other metadata, and
A very nifty Retouch tool has appeared within the Develop module which allows you to adjust Brightness, Saturation, Exposure etc with the use of a brush.
In fact the list of improvements just goes on and on. All in all I'm quite impressed and I hope that everything I've seen stays in the final release, which I believe is due out around Autumn.
If you want more info on the changes, I've found this blog post by Uwe Steinmueller here: http://www.outbackphoto.com/CONTENT_2007_01/
section_workflow_basics/20080402_LR20Beta/index.html which contains loads more details and screenshots