They've also incorporated AJAX calls to the Twitter database (much like the new FriendFeed redesign) to increase the speed of the web interface and reduce the amount of manual refreshes required to get an updated view of things.
They've also very much improved the profile editor, with a number of color choosers and editable areas so as to make your old background images and designs blend more adeptly with the new site color scheme.
Of course, the big question: Where's XMMP and Track functionality? Twitter has an answer:
This hardest thing about doing a redesign like this deciding what not to tackle. I'm fairly certain that much of the feedback to this will be, "What about...[your favorite feature request / annoyance]." Please be assured the changes we've made here aren't the only things we want to (or will) change. They're not even, necessarily, the most important. The scope of this project was limited to light-weight front-end work. We have whole other teams working on back-end changes and more fundamental functionality changes (which, as mentioned above, this is also laying the groundwork for).