LoL does not meet MMO criteria. It needs hundreds of players playing consistantly, in a persistant environment. They have instanced games with seperate character progression that contain a max of 10 players. Does not meet t he criteria. Also, Summoner skills/levels do not suffice for the type of progression MMO's have.
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Its is a muiltiplayer game
and its online"
If you went purely by that, you can call any multiplayer game with a mass amount of players an MMO. Aka COD, Halo, SC2, etc.