Most of the problems with new versions of Windows stem from driver issues. It is pretty much impossible for MS to test things on all of the hardware that people are going to be running their products on, so you end up with issues that take a while to solve.
Apple doesn't have this problem because OS X runs on a limited amount of hardware that Apple controls almost totally. So, Apple doesn't have to support near the amount of hardware the MS does, nor does it have to rely and third party hardware manufactures to get their ducks in a row when they release a new version of the OS.
What this means is that Apple has fewer problems both with initial testing, in getting the product out the door, and in patching bugs later on, so you can pretty much bet that the initial release will go pretty smoothly.