[untog]

Could someone explain what motivates Microsoft to try and "completely reimagine" their OS every time they iterate?

They don't, though. Vista was, cosmetically, a big leap from XP, but in terms of actual UI it barely changed at all. Start button is still in the same place, maximise, minimise, etc. All still there.

I do really appreciate the little touches MS add, actually. Drag a window to the top of the screen to maximise, to the left to use 50% of the screen, shake the window to minimise all the other ones... I use that stuff every day, and miss it when I go home to OS X.

[doublextremevil]

I use better touch tool to do pretty much exactly that on OSX (along with multitouch gestures) http://blog.boastr.net/