Hang on, Fallout 2 is "gritty"? With its stupid pop-culture references and goofy fourth-wall breaking dialogue, its ghosts and chess-playing scorpions and talking deathclaws and cartoon gangsters? Pull the other one. None of the Fallout games are "gritty", whatever that's supposed to entail. They're not exactly Threads or The Road...
Anyway, I'm not hostile towards the idea of remaking the originals, but let's face it it ain't gonna happen.
edit:
Fallout 3 is rated M for some reason, I guess because once in a blue moon a character will say a swear :rolleyes:
Or, just to hazard a guess here, maybe it's all the violence, gore and exploding heads?
Honestly though, the idea of a "mature" computer game is pretty much an oxymoron. It's an inherently immature medium, and all the better for it.