I think the main problem with Linux is that it doesn't have a killer app (the Mac's is everything Just Working, and its superiority at graphical processing). There's nothing you can do in Linux that you can't do in Windows, so basically you're moving to an OS where you have to make sacrifices.

The only advantages I can think of in Linux are:
* Free as in speech and collaboration (though most people really don't care about politics, they just want a computer that works)
* Shell-based power tools - which you can use in Windows with Cygwin
* Lack of viruses and spyware - which are a non-issue if you're educated enough on computers to think of installing Linux in the first place and
* The fact that it costs nothing (but let's face it, how many people just pirate everything....)

I honestly want Linux to succeed and improve. But those are the reasons I think it's stalled a little so far. It needs a killer app or at least a killer ability that is attractive to the masses.

FWIW I have a Windows box (main PC), Ubuntu Breezy box (bedroom PC) and Powerbook. So I do use each of the big three OSes.