actually declined by 600,000 users after the release of the latest expansion, Cataclysm. Two months before the release of Cataclysm, WoW subs hit 12 million. Today, the number is 11.4 million.
Blizzard president Mike Morhaime says that players just blew through the content faster than ever, because the players are getting better. So Blizzard intends to speed up the release of new content, though exactly how they plan to do that isn't mentioned.
Not everyone agrees with that explanation for the decline, though. This long-time player just got fed up with the incredible rise in difficulty level. It's certainly got to be a difficult task to keep the experienced players happy while still attracting new players. It's true that WoW at the high end has become a very technical game, requiring a high level of knowledge of all the spells and items and their interactions so you can optimize your character. That prospect is rather daunting for a new, casual player.
Still, I remember that one of Blizzard's design goals with Cataclysm was to bring new players into the game. I'm sure it did, to some extent, but clearly they are not bringing in new players fast enough to replace the old players who are leaving. WoW needs to make sure they are solving the right problem here. Is the real answer to make the shift to free-to-play?
I'm sure that's being considered, but it will be a difficult process at best. It will be interesting to watch the progression...