Okay, if I could just clear up some of the discussion here, and throw in the opinion of the general public, as reflected by Gamerankings.
M rated games on the whole get higher ratings than the average of all E rated games? Why? Maybe the hundred thousand stupid kids movie rip-offs? The crappy licensed games based off some stupid TV show? These make up a lot of the E rated games out there. While there are many (a lot made by Nintendo) that are really quite great, all those crappy licensed games really bring down the average scores of E rated games. M rated games, on the other hand, are targeted towards adult audiences, who (hopefully) will not buy a game, even if it's licensed if it sucks. Hence, M rated games get higher scores on average.