The universal appeal of MM lies in the fact that it doesn't get anything wrong rather than getting anything spectacularly right. It's a game to be enjoyed rather than a game to be loved. Very few people would find anything to object to with this game- and a major reason is that it's not licenced. Players don't feel alienated or confused by not having seen a particular movie, not listening to a particular band, not reading particular comic books etc. The trade-off is that MM is a lot of people's second-favourite game; it will never have the kind of devoted fan base that Centaur, STTNG or LOTR have.