definition of "MMO" is changing at an increasing pace these days, to the point where many people argue that the very idea of defining an MMO is a thing of the past. However, a defining factor that has stayed consistent through all of that is the question of how many people can play together.
"The more the better" is the commonly held standard, but not everyone prefers to play that way. An increasing number of games offer quests created for solo play, the ability to create solo play skill sets, and even the ability to play through the majority of the game alone if you prefer. While grouping isn't obsolete by any stretch of the imagination, the solo player isn't left out in the cold either.
So what is your preference? Would you rather go it alone at your own pace, or are you there for the company of others as much as the gameplay?