You clearly haven't looked at Mac OS X
With Xgl, the problem is that it's suffering from a big problem: 2 different gui frameworks: gnome and kde. A gnome app will work on kde, though the fonts are BIG as they use a different DPI standard. No matter what you do, it's hard if not impossible to get that right.
The video shows some nice gimicks, but does it really bring more functionality which lets you do your work faster and more productive? I have my doubts. For starters: I don't want window gimmicks, when I click the minimize button, the window has to go immediately, no animation or anything. Any resource eating goo has to go, every cycle in my PC has to be utilized by the apps I'm using, not the OS. The OS should be there to make running the apps possible, that's it.
You know what Apple will do soon? Offer real virtualization on a single apple box: real time MacOS X AND Windows XP running at the same time, next to eachother, not via virtual PC slowness, but truly virtualized, so the performance degration is minimal, if at all. That'll be the real future, vista killing material: have 3 or more OS-installations, each tailored precisely for your task you want to use the OS for.