[KaVir]

Unless I completely misread what DurNominator said, he's not comparing MMOs with MUDs, but rather MMOs with Computer RPGs (CRPGs).

Features found in a typical CRPG include things like character development, freedom of movement, multiple quests, combat based (at least partially) on character ability, a well-developed fictional setting, etc. However many CRPGs are exclusively single-player games, and even those with multiplayer support are usually a primarily single-player game.

I suspect that's where 'MMORPG' comes from - they're not just Multiplayer RPGs, they're Massively Multiplayer RPGs that (unlike most CRPGs) can only be played Online. As such I would consider MUDs/MMO(RPG)s a subset of CRPGs. I guess we can all click the 'Roleplaying Enforced/Mandatory' checkbox now, as pretty much every mud fulfills the criteria of a Computer Role-Playing Game.