Retail games offer three things, innovation, refinement and artistic appeal. A game needs to have either an interestingly new game design idea in action, a refined version of an extant idea, or significantly increased artistic appeal. Without one of these three things, a game will not sell. World of Warcraft, has an extremely stable subscription base made up of people who have put so much time and energy into their characters that they won’t give them up if they are not completely bored. Thus, Blizzard needs only to give the players the minimum amount of content, with sparse streamlining.
Admittedly, this is a cynical view of subscription gaming. But it is logical; why would a publisher want to provide more development money than absolutely necessary? There is no MMO threat to WoW; nothing is going to unseat the monster of the industry. And WoW certainly is the monster of the MMOs because of all the development that has already gone into the game. That said, it is logical to spend the bare minimum amount of money, to rein in the developers as much as possible, to add as little as possible.
The bleakest possiblities in World of Warcraft have not happened. The development team still works hard, there are still new game design elements, and it's clear they care about their product. Wow has not, however, seen a major game design innovation since Burning Crusade. Their upcoming expansion will tell the gaming community much about the future of the giant MMO. Until then we can only surmise.