The problem though is going to be corporate customers. The ones with thousands of desktop systems that do pay. Big corps tend to be conservative about IT upgrades, and by giving Windows away MS would be sacrificing that revenue stream. They're probably reluctant to do that.
Of course, they could just drop the price of the Home Edition (or whatever they're calling it today) to zero and charge for the Pro one. But then they need to make the home edition good enough to be useful, but not so good that business would be happy using it. That's not compromise that's worked well for them in the past.
Actually, it's quite straightforward: the Pro edition can join domains, while the Home edition can't. This by itself will work quite well as a differentiator. Big businesses aren't going to give up Active Directory and Group Policy to save a few bucks on license fees, while home uisers (and some small businesses) won't give a damn.