Well, you can't run windows binaries on GNU/Linux system, and many new users try to install .exe-files, without luck. For me personally, I found the directory structure unusual. I didn't quite understand how to access floppys and CDs without any a:, d: etc. (I came to learn that the *nix way to do this is far superior
GNU/Linux can run on so many different systems, because the user have full control over his/her environment. There is no people in a central place, say Redmond, that in cooperation with hardware vendors decides how great computer you should have to run the new operating system. If you GNU/Linux system is slow when running KDE, you can run something less resource heavy, i.e. fluxbox or lxde for a little more functionality.
Well, in GNU/Linux you can compile your kernel to include support for mostly all hardware that's available. Windows have only basic build-in support, and you expand with drivers. You can obviously do this with Linux aswell, like Nvidia does for it's binary support for 3d acceleration. More detailed differences regarding Linux and Windows I will leave for someone else
Edit: Did I just do someone else's homework? D'oh..