Typically, a Massively Multiplayer game is defined as one where hundreds or thousands of people exist, online, in the same environment, at the same time. Games that require you to create or join an existing 'room' or 'server', limited to less than 50, or even 10 people, are not 'massively multiplayer'... they're merely 'multiplayer'.
So I wouldn't classify Diablo/Diablo 2 as MMOs. Not at all.
Guild Wars is a sort of hybrid. Since you can co-exist with hundreds or thousands of other players, in certain areas of the game, it does have some MMO traits... but for the most part the game limits the players to 8 or less (for any real gameplay).
Typical MMOs are those like Evercrack, World of WarCraft, Asheron's Call, Dark Ages of Camelot, Final Fantasy XI, etc.
