The whole "Vista is nothing but XP with a fancy new UI" canard has been provably untrue for a long time, but there hasn't been much literature explaining why -- at least until now. TechNet Mag has an article by Mark Russinovich (of Winternals and Sony rootkit-busting fame), the first in a series, about what's new in the Vista kernel and why. It's pretty technical, but Mark breaks down some the more difficult-to-understand stuff for a reasonably sophisticated reader. I plan on coming back for future installments.
I saw that article earlier tonight and it is excellent. Also, Mark's website at Microsoft includes links to video or powerpower presentations that discuss Vista kernal changes, memory management, and other similar topics. Here is a link:
http://www.microsoft.com/emea/itsshowtime/sessionh.aspx?videoid=340