>>10
Shiny window manager?
Mac OS X doesn't rely on the ancient X11 system and the popular X.org implementation.
And FreeBSD? The kernel itself is only part BSD, with significant code from Mach. The IOKit driver system is completely unlike the traditional Unix/Linux model. The GUI stuff is all in Objective-C, which is used almost exclusively by Mac OS X.
>>11
Mac OS X's compiler has more optimizations enabled by default than standard gcc.