I think WoW is on the list as the token MMORPG. It's there for the same reason Oblivion is: it's new, dumbed down and popular! They should've put Ultima Online and/or Everquest there instead, seeing as they were much more influental in the genre and actually innovative instead of just upping production values and polish on the same old same old. Or the original Neverwinter Nights, Meridian 59, the Realm etc etc but innovative as they were, I don't really see any of them as the best PC games ever.
Overall I'm very much surprised. I expected nothing less than the last 5 Maddens in top 10 and a full top 25 of EA's games. There's a lot more genuinely good games than overrated crap on the list. Who would've honestly thought IGN, the poster child for all that is wrong with gaming journalism, of all people would name Xcom as the best PC game ever? Still, nothing from the Ultima series!? It's not like anyone actually takes these lists seriously, especially coming from IGN, but not mentioning any of the Ultimas seems like a major oversight.