This is one of the misunderstandings that I want to see resolved between KWin and Mesa/Xorg devs. We need to know what the other side means. E.g. if I say "We need to improve Compositing", I mean improving User Experience and if driver devs read this as "We need to ship more eye candy", we have a problem :-)
In the past with just Compiz it might have been that especially in the beginning there was a large emphasis on eye candy, but this has never been the case with KWin.
The real fact of the matter is that both windows and osx have a suprior composited desktop experience right now, and better game support.
Composited desktops in linux are extremely random performance and stability-wise, probably partly due to driver crap but its a mess compared to say aero or quartz which perform well on every machine I've used them on.
On my machines scrolling is total crap with compiz enabled compared to windows. Kwin just has horrible performance all around :/ Haven't been able to really test gnome 3's mutter because gnome 3 doesn't work well with ati right now.
What I do love about linux is how configurable everything is, there's not any options for windows aero. But performance, stability, and usability wise linux has a very long way to go in this area.
I think a proper composited desktop experience is very important for a modern distro aimed at being user friendly.
But really, some people doesn't understand that desktop isn't tablet/phone. Or I'm the one being ignorant?
Then you should take a look at Windows, KDE and OS X. Simplicity means something much different out there.If anything, the trend seems to be towards simplification and bringing the mobile user experience to the desktop, at least that's what I am seeing.