This paragraph from John Gruber's Daringfireball.net post on iTunes and it's potential 64 bit"ness" sums up perfectly the difference between Microsoft Windows and Mac OS X:
The single most remarkable thing about Snow Leopard is that it is smaller than the previous version of the system. It is an operating system that is — and, going back to its roots at NeXT, has always been — evolving, not just growing. Apple doesn’t just add to it. They prune. They churn. And the track record shows that when it comes to ushering old technology out the back door, they err on the side of too soon rather than risk letting it linger too long. Apple worries about the way things should be far more than it worries about continuity with the way things used to be.