Windows - pro - pretty much all software works on it.
Windows - con - usually 3-5 years behind the competition, v1.0 products *really* suck. Just not cool.
OSX - pro - proprietary hardware allows Apple to have reasonable reliable software/drivers (not as much variety of hardware to support). Beautiful hardware, great UIs. Pretty cool.
OSX - con - a little pricier, not as much software available, but pretty much anything you want is catered for.
Linux - pro - open source so well eye-balled, Bloody cool.
Linux - con - not enough software available (no there isn't! Tell me which video editing software I can use?).