If you've got a wired Xbox 360 controller, it's actually pretty easy. The controller just hooks up to your PC via USB, so plug it in, and you can then use a program called Pinnacle Game Profiler to recognize the controller, and even attach keyboard and mouse functions to the controller's keys. The problem here is actually making sure you have access to everything you normally would (a nearly impossible task, and probably the reason why WoW hasn't come to a console). But if you can do it, feel free.
The problem is if you have a wireless Xbox controller. Unlike the Wii, apparently Microsoft doesn't use the normal Bluetooth standard for its controllers, so you either have to buy a controller specifically for Windows, or an adapter to hook it up. And after you do that, you still might have to download the Pinnacle program to set it up right-- I've got a wireless controller right here, but I'm not about to spend more money for an adapter when I like using the keyboard anyway.
Have you had any success getting the Xbox controller to play WoW? I like the controller and all (although that D-pad is horrible), but why would anyone want to abandon the mouse and keyboard in the first place?