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If we look at typical themeparks mmo in its endgame, we only see players waiting in "lobbies" (cities, entrances to dungeons, etc) to join a "match" with others in a enviroment completely separated from the rest. In this case, what's the difference between these mmo's endgame and a normal single-player game in multiplayer mode? I particularly cant see any, since most modern single-player games also incorporate elements of player "progression" in their multiplayer modes.
Anyone could cite more differences that can allow us to define which game is or isn't a mmo?