For those wondering, Diablo 3 will only support a certain amount of players per Game. That disqualifies it as an MMO which consists of thousands of players in a persistent world on a single server. The amount of players per game in Battle.net 2.0 has not yet been determined, but Diablo 3 Community Manager Bashiok explains a few things about this topic:
Bashiok: MMO’s are generally categorized as being able to support hundreds of thousands of simultaneous players in a persistent world.
While Battle.net can certainly support that many players at one time, the lack of a persistent world and restrictions on how many players can be within each “world” (game) would keep Diablo III from being categorized as an MMO.
One of the larger things to think about when balancing game sizes is that with mechanics that encourage players to stick together, only so many players can be doing these huge over-the-top type abilities without it turning into a visual mess.
We’ll settle on a group size that allows for the most amount of players while still keeping the games fun, and sane.
