For most people, the two biggest advantages of a PC over a Mac are that Macs cost more, and you can't play (most) games on a Mac. Most Mac owners I know either have a separate gaming rig or dual boot to Windows just for video games.
Today marks an inflection point in the Mac vs PC war: Steam has been ported to Mac! The only games I play on a PC anymore are those from Valve (Left 4 Dead, Counterstrike, Half-life, etc) and from Blizzard (Starcraft, Warcraft, etc). Most other games are better experienced on a console. Well, both of those sets of games are now going to be released for the Mac on the same day as the PC!
As someone who owns Microsoft stock, this is a big problem. You do not want an OS where your main differentiator is that it's cheaper, or to rely on mass-market inertia. My computer use is split amongst internet use, coding, creativity software, office software, and video games. If I were to buy a computer today, for the first time, I would actually consider a Mac. For the first time, Mac has achieved parity with PC across my usage scenarios.
This is a dangerous time for Microsoft.. tread carefully.