Window dressing
It's probably fair to say that the playing field is pretty level for both Microsoft and Apple from a kernel perspective–after all, hardware is hardware, and the bigger problems come not with driver compatibility with some cheap-$#it audio IC from Singapore (you can always trade off performance for stability), but with overall stability and sustainability issues.
Apple's first version of OS X was not anywhere near ready for prime time (10.2 is probably a minimum for most people) at its launch, and many prominent developers did not come on board until much later, as developer resources started to flesh out.
It's worth keeping in mind, however, that Microsoft, at the top of the heap, has far more to lose if it can't develop new and exciting products, from the OS on up.
FWIW, I just removed Vista from my MacBook Pro laptop and replaced it with XP, as half of the things I need to work, don't.