I thought this was interesting and relevant, from
Wiki:
- MMOGs create a persistent universe where the game continues playing regardless of whether or not anyone else is.
- MMOGs host a large number of players in a single game world, and all of those players can interact with each other at any given time. ... Non-MMOGs ... are usually played on private servers.
- Neverwinter Nights (2002) and Diablo II are usually called online role-playing games, (RPGs) but are also sometimes called MMORPGs (a type of MMOG). Guild Wars has been called an MMORPG, but most of its gameplay involves small groups of players in private areas. The game's developer prefers the term "competitive online role-playing game".
The way I see it an MMO must have a persistant game world with a large number of subscribers on a single (or multiples thereof) company-owned centralised server.