After yesterdays post about the Diablo 3 announcement, I found that I was getting a fair bit of Google traffic from people asking if Diabl0 3 is a MMO.
The simple answer to this question is, no. Diablo 3 is not a MMORPG.
The slightly more complex answer is that the lines between MMORPG and simple multiplayer games are blurring, and have been for quite some time.
Games like Guild Wars allow large numbers of players to gather in various areas, and then group up and head into instanced zones for the actual gameplay. The areas where players gather are much like the lobbies that services like Battlenet provide. Places where players can find others to group with and play the game areas. The only real differences are that the gathering areas are graphical zones and that they often provide NPC services like quests and shops.
If we use features like big open zones where players encounter each other randomly as they play to define if a game is a MMORPG or not, then no, Diablo 3 is not a MMO. But then neither is Guild Wars, even though most consider it to be one.