While I agree that there has been progress on the unified notification, there has been 3D accelerated composited desktops (which IMO look better than Aero and Aqua) for a while now in the form of Compiz. Yeah, it may be tacked on, but in many ways, it can integrate well with Metacity and Kwin. I even still use it in KDE4 over it's built in compositing as it adds more options, and generally runs faster (for me anyways). I never really understood why neither desktop just didn't stick with that instead of reinventing the wheel.