Apple completely re-write OS X from scratch... They didn't make minor changes...
And Operating Systems get exponentially harder to develop, particularly if you have unrealistic backwards compatibility goals.
I think Microsoft should take its lessons from Vista and for their next big release drop all backwards compatibility before Windows XP (e.g. no Win16, MS DOS, POSIX, and OS/2 etc). These old components also offer the biggest security headache... They were written
in the dark age of computer security and thus cannot be trusted.
Microsoft might do what Apple did in OS X and for the next major OS release add a virtual machine that will run all the old junk, while removing it from the primary OS its self.
Windows XP has twenty dlls last updated in 1998 and 1999. It also has 1,616 dlls that I've
never accessed since I installed virgin XP. On top of that there are 300 executable (exe, com) files that I have never used since installing Windows XP orginal.