I'm taking an accelerated class next week for Windows Server 2008 and Active Directory in Charlotte, NC -- part of the class itself is a pretty in-depth self-study component.  Covers everything from the marketing and positioning that we're doing along with the technical details.  Whitepapers, PowerPoint slide decks, WebCasts (both public and private).

One of the links that I stumbled across while going through the AD materials was this document outlining what each of the Domain and Forest Functionality Levels gives you.  Previously, I know knew at a topical level what each level turned on - but this is pretty good information.

You can find it up on Microsoft TechNet:

Appendix of Functional Level Features
http://technet.microsoft.com/en-us/library/cc754918(WS.10).aspx

Edit 4/2/10: Rob Bolbotowski let me know that the TechNet article had been removed/moved/transmogrified and so the link was dead.  Updated the link for the new content.  Thanks Rob!