Well, considering the direction most MMOs are taking, the difference is starting to be really small between a MMO and `non-massive` multiplayer game.
I mean, the ´world´ is mostly a hub with PvE and PvP content inside instances. Diablo (or FPS) are not that far from that with battle.net (steam?) being the hub.
In theory, MMOs have an open world where the amount of players isn't limited. But it's often limited by technology. AO kicks players out of the tower field and, even then, the game is barely enjoyable.
AO is definitely closer to a standard multiplayer game than EVE with its 1-2k pilots wars.