Well, first off the parental controls that everyone threw a fit about not to long ago. Then add to that the Xbox originals and social networking features.
The new news is the news feed from xbox.com (maybe at some point we’ll be able to add our own RSS feeds) and you can get yourself a full profile with added info. Movie trailers are finally going full screen and more codec support for enhanced video (is the 360 getting the same DivX compatibility upgrade as the Media Center Extenders?).
Lastly, dashboard navigation is said to be easier with dedicated Game Store and Video Store buttons. We’ll find out everything in a few days on December 4th, until then, we’ll just keep making educated guesses.