Vagabond: What makes a PC game?
Simple.
Innovation.
+1
No other platform can tolerate innovation, experimentation and failure like the PC. In the time between innovative console releases like Frequency and Katamari, we have dozens of indie, shareware or open source classics like Gish, N, and Karoshi.
There are no barriers to publication, which is a feature that doesn't exist in ANY other major gaming platform (and, for this reason, the PC conveniently acts as the proving ground for many of the titles in XBox Arcade, WiiWare, etc.).
Another feature exclusive to PC games is moddabilty - the ability to pick up someone else's game and extend it further with your own ideas (the concept that spawned Counter-Strike, Team Fortress, and several other A-list games). And if that's not possible... you can use the game's data files and write a completely new engine to work with them (ScummVM, Exult, D2X, etc.)
Thirdly, simulations are nigh-on impossible with anything other than a PC. Not only does a PC have the 100+ buttons and multiple axes required to operate a flight simulator, but the PC architecture means you can even build your own cockpit to play in.
Finally, PC games are able to cater for demographics which most consoles can't reach (games for children, puzzle and casual games for the non-gamers, or web-games for people who don't have a computer of their own), so there are plenty of titles in each category. PC games also have a unique ability to have runaway successes with gamer demographics that weren't previously known to exist (The Sims, Bejeweled).
PC games are continuing to innovate and differentiate themselves - it's just that now we need to look at indie developers or smaller publishers (Garage Games, Stardock, Strategy First) to push the envelope, instead of the publishing houses we used to rely on.
My question is, what makes a console game "exclusive", besides big buckets of cash?