I think the 3 points of Jastiv resumed well my opinion.
MMO have been created to give something really huge and unique to the players.
(Of course take the numbers with a lot of salt)
So in terms of numbers there should be at least 1000 players on a server. Why 1000 ? because a MMO with less then 1000 players feels really empty or small even for a MMORTS.
It may be possible to have 64 players at one location. 64 players because it has been possible to have 64 players at one location since (at least) the tribes series so there is nothing huge if it's not the case (and tribes is really old now and it's not a MMO). Of course it may vary depending on the type of the game.
Not instantiated world like Diablo as it has been said Jastiv, and so the world must persistent in some way. Again if you look at my definition if the game is only a multiplayer game with a lot of players that doesn't sound right. The persistent aspect of the MMO goes well with their goal (give something huge and unique to the players). It's so huge, it becomes a (virtual) world, which is very appealing. It's like a player dream in a way.