Sorry, I got to this party late, but it bears repeating that the Blizzard name probably sold more copies than the actualy quality of WoW.
I say this because that Blizzard name was enough for many people to buy their first MMORPG and we all know the many effective incentives the genre has to keep you playing after that initial purchase. Then you have the social aspect of the game that feeds off the huge subscriber base. If all your friends are playing it and pressuring you to play, what do you think will happen?
I have great respect for Blizzard, but I also recognize that they garner a huge traction from their name at this point. They had one good idea in their existence with Diablo; the game that influenced every other CRPG made since (mostly for the worse). Warcraft 1 and 2 were derivative RTS games. Starcraft was the game where Blizzard hit upon its winning formula I like to call the three Ps. Pilfer(from other games), Polish and Playtest. They used this extensively in Diablo 2 where they followed suit and made a clone of their own game but still found great success. Warcraft 3 same thing. Instead of the innovative RPG/RTS it started as, they backtracked to a standard RTS system and borrowed from the rather dull RPG/RTS glut of the time.
Then we get to WoW. You could seriously point out nearly every mechanism in the game and I could name its origin. All Blizzard managed to do was 1) have the Blizzard name and 2)do the obvious thing and appeal to the casual player, a market they had tapped with Diablo 2. In fact, it is amazing nothing like WoW came out sooner, all the ideas were already in place.
So in summary, I respect Blizzard because they bring some fundamentally sound design principles to bear, but they long ago stopped ranking as a truly innovative company. Now, excuse me, I have to go level my human warlock.