Mostly security enhancements, according to Microsoft Watch.
"New" features include tabbed browsing and support for transparent PNGs. No comment yet on whether full support will be given for CSS2 - *sigh*.
Curiously, there's also talk that IE7 will feature an integrated news aggregator, which can only help to fuel the popularity of the RSS medium - there are very many PC users out there who use email and browse the web daily, but have never seen the need to install a separate news reader. Having such a tool pre-installed on their new PCs will put the technology in their face and make it easier to give RSS/Atom feeds a try.