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Heftier than XGL/AIGLX, it seems.
It also does a hell of a lot more than XGL/AIGLX. In any case, by the time Vista is out later this year, anyone who has a system that is overall Vista-worthy will have at least a Radeon 9XXX/GeForce FX original or equivalent. Even Intel's GMA950 is capable of a lot of Vista effects.
Vista requirements are exaggerated. So it won't run on your Voodoo 3 ... boo hoo.
It also does a hell of a lot more than XGL/AIGLX.
Such as? The only real difference I've heard so far are pixel shaders, and I don't see why these couldn't be added to XGL. One thing we know is that the open-source desktop has developed faster than proprietary ones, so we'll see where we'll be when Vista comes out, and then a year later...
In any case this is quite off-topic. I'm personally happy to see cooperation between the two projects.