Black magic. I'm doing lots of work with development versions of the kernel, PolicyKit, HAL and X and I need a clean environment. Ubuntu has lots of black magic, which is great from a "just works" perspective but bad when you are trying to hack on core bits of the stack.
Don't get me wrong, I still think Edgy is great, and would wholeheartedly recommend it for someone new to Linux. For me, Fedora is just right.