Just compatibility with everybody else I know who, for some unknown reason, uses Windows computers.
Plus the numerous times I've tried using a web page only to find out it isn't Mac compatible. Or the numerous times I've had somebody tell me about a great application only to find out that it's Windows only.
I'm upgrading to a G6 as soon as somebody invents it, but I got the Windoze PC for now so I can actually walk into a software store for a change and actually find something that works on my computer.