Apart from the actual change of colors and layout the new tabbed interface is the biggest user interface change. Navigation is far much better with this and with the breadcrumbs.
The search is the most improved feature, now with auto-complete and more indexed content.
The MSDN library tree seems to be much faster, really nice for users (like me) who uses the online help instead of the full DVD install of MSDN library (I always forget to update it when a new DVD arrives). But it is still reloading the whole page and tree for every click in the content on the right!
I think it’s a great update to MSDN, although I have some troubles connecting to it right now…