There are so many ways to define failure. Doesn't it sometimes seem that each new MMO tries to come up with its own innovative formula, some new and novel way to drive their game headlong into the abyss?
Star Wars Galaxies was a great game until Sony Online Entertainment got WoW envy, dumbed down the game, and lost their subscribers. Warhammer rushed their game to market, delivering a horrendously buggy and unbalanced MMO (The PvP was great, though.). Dungeons and Dragons Online (DDO) had fabulous PvE instances, but not much else to keep you busy. Champions Online was fun until you got to the end and didn't have much to do. And Star Trek Online was rolled out only half-finished, ignoring the fact that Star Trek fans wanted to play Klingons.
In an alternate universe, all of these MMOs might have been great. Each of them had hordes of excited fans and MMO gamers anxious to play their game, and yet each managed to fail in its own way.