Most Intel processors fail to run Windows without being very annoyingly choppy (Windows XP, actually. Older versions are fine). I don't think Mac OS X would be bareable on most x86 processors.
If Apple included an x86 emulator with OS X, people would stop developing native OS X apps.
Mac OS X has drifted away from being compatible with Linux apps. Binutils isn't in OS X! Guess that makes porting stuff a little bit harder. People who say "why would you want linux when mac os x is linux!?" don't know what they're talking about. For one, Darwin is based on BSD if I remember right, and secondly, they have never tried to compile stuff that needs binutils. Also, Apple's X11 is slooooow, especially on my G4/466.
I just bought a Powerbook. Why didn't I buy a PC? OS X is really great. Aqua is very, very, very quick. I find OS X on an average machine much quicker than Windows on an average x86 machine.
My Grandma has a Celeron 1.7 Ghz and it can barely run Windows XP home edition. A window refresh usually takes somewhere around 5 seconds, honestly. Windows sucks, and it's making x86 look terrible, whether it really is or not.