Comparing XP to Windows 7, there are really awesome reasons why I should make the switch. Improved performance, graphics, the pin bar, windows search, easy network management and a plethora of other useless/useful features.
There is a precedent for this in Leopard, where a significant UI overhaul and some other features were actually announced and demoed many months after the initial showing.
(Also: it's amusing that you skipped Vista.)
I really like Mission Control, but I want to hear about a finder that's finally good (tried the alternatives, didn't like any of them) or resolution independence.
None of these features had the whiz-bang appeal of past innovations like Time Machine, etc.