The Engineering Windows 7 blog has a list of thirty six "visible" changes coming in the next version of Windows 7. Here's a quickie list of the ones worth knowing and hotly anticipating or whatever:
Aero Peek—where all the windows on the desktop become transparent except the one you're highlighting—will now work with Alt+Tab. Just pause for a second on the thumbnail you want to scope out. Pretty awesome, since it means less mousing, more peeking.