AFAIK (I don't have the source right now), Apple originally considered using X11 for the display server on OS X (in fact, early versions shipped with a version

AFAIK (I don't have the source right now), Apple originally considered using X11 for the display server on OS X (in fact, early versions shipped with a version of xf86 that ran natively as it does on other *nixes), but after deciding on all the compositing and prettiness features that they wanted, they would have torn so much out of X11 that it wouldn't be remotely compatible anymore, so they wrote Quartz 2D. Later, of course, things like Compiz came along, but they've always seemed like weak hacks compared to things like Quartz. Hopefully wayland will change everything.