It's just not set up the way I want, with things like focus follows mouse, and it's harder to customize than Linux with a good window manager. I also prefer how Linux package management works; that it's an integral part of the system rather than something bolted on. I do like the screen and keyboard on the Mac, with the exception of the lack of volume buttons.

I also like the open nature of Linux. It's fun to be able to look into the source code for pretty much anything.