Lots!
See:
Converting to Access 2007
at:
http://allenbrowne.com/Access2007.html
The article covers:
* What's Good - Useful new features
* What's Mixed - Both good and bad
* What's Gone - Features removed
* What's Fixed - Old issues solved
* What's Broken - New bugs
* Configuration - Setup options for developers
* Compatibility - Issues with previous versions
* Links - Technical info for developers.
