Just started truly exploring but 2 immediate favs:
1) Drap and drop media uploader. Hot stuff. Also, the uploader no longer designates certain icons for certain file types; it’s all a single icon and after you drag&drop, WordPress does the thinking for you.
2) Comments interface. It could possibly just be the styling of comments in the WordPress twenty-eleven theme, haven’t checked in other themes yet, but I really like it.