"The windows all stack up when you press Alt-Tab."
This looks pretty darn cool, but isn't it kind of the opposite of what should happen from a usability perspective? If I'm trying to find a particular window buried somewhere, I want them to UNstack. (like Expose in Mac OS X 10.3)
This 3D switcher, while offering better visual cues about what you're switching to than the old alt-tab behavior, is still a linear form of access (assuming it doesn't let you click on one of the windows you see at the bottom of the stack to immediately switch to that one.) Expose, on the other hand, allows random access -- click any window to switch to it.
And, despite being 3D in appearence, the Longhorn switcher organizes information along 1 dimension (the "Z axis" of the window stack), whereas Expose organizes windows using both dimensions of a 2D screen.