M Rated games (or their counterparts in countries not lorded over by the ESRB) tend to be a little bit more realistic in presentation, as it's usually the blood and language included in the emulation of realism that earns it a 17+. Also, there are a few games that I find myself ignoring because they're not M--not because I'm a gore junkie, but because it means that a potentially gritty subject was sugar coated to get it past the ESRB and parents who don't know anything about games and into the hands of 12 year olds. For example, I had a mild interest in the concept behind Dark Void until I found out that it was T (the fact that it was apparently a shitty game didn't help it much either), because that meant that the developers had passed over some potentially brutal takedowns--if a game is going to advertise the ability to hijack vehicles, it needs to be a little bit GTA about it and offers some sort of bloodlust reward for the action, more than just a small QTE.
Also, all too often the games with intriguing stories are too mature themed for the kiddies--Heavy Rain and Bioshock, to provide some examples, are two games with interesting story ideas (although I can't conclusively say that Heavy Rain has a good story) that would NOT have worked with a T rating, because hunting serial killers and trying to survive in an underwater city filled with homicidal maniacs are going to give the little ones nightmares.