The default skin is not an improvement over 1.5. (I'll be picking a different one.)
The close boxes on every tab require you relearn your behavior, if you're a mouse navigator. I really want to revert this.
Tab state is saved across sessions.
Optional Google Reader integration with the previously useless RSS live bookmarks.
What I've Changed
I've dropped Torisugari's Bookmarks Synchronizer in favor of the more transparent Foxmarks (syncing with my own WebDav server). Oh, how I wish Foxmarks stored the favicons like Torisugari's Add-on did. C'est la vie.