Re: Base theme?
Standard WordPress support (not just MU)
Activity stream threaded commenting.
Ability to mark an activity stream item as a favorite.
Activity stream item permalinks.
Advanced activity stream filters.
New default theme with improved usability and customization options
There have been some template tag changes (incl. the template tag loops), I would advise you get a copy of the 1.1 default and 1.2 default theme, look at the changes and apply them to your site. There’s no documentation written yet.