I got a look at the early Sun OS offering (don't ask how or I'll have to kill you) -- it was extremely cool, with flip-over 3D windows and an awesome UI.
It was also buggy and crashed 3 times during the demo, which was given by the ponytailed, jeans-wearing VP of Technology.
3D windows don't impress me. Okay, so I can rotate windows into the background... encouraging me to open more windows, which means I've just lost track of everything I've got open, and I have to go back and flip all these windows again to find the one I'm looking for.
Sometimes I really wonder how much sway the UI designers at OS vendors have... assuming they have UI designers at all.
Though, to be fair, Vista's implementation is going to be far worse, because of the taskbar.