Assassin's Creed can be a little bit mature at times, but is nonetheless a very well done game, and there is a way around almost all of the bad content.
Language: This was really the main thing that bugs me about the game. UbiSoft really put a lot of it in there. But there is a common solution. Turn off the stinkin' volume! And either way, the usually swear in Italian, so yeah, turn off the subtitles.
Blood: Ah, HERE we are, the one that every M rated game seems to have. This is easier to solve than language. You simply pause your game, go options, go to general, and there is a blood on-off. (For those of you who wonder, I've also seen this in most Call of Duty games, and it also turns of the language.)
Sex: This one really worried me at first. No matter how fun a game is, I never play something with too much sex involved. But you know what? All it is is seeing a back for 15 seconds, which you don't even have to do! It's part of a minigame, that you don't have to do. All in all, the sex is nothing you don't see in the average PG-13 movie.
Violence: Yeah... There's no real way to turn off violence. You can minimize the amount of violence by simply not engaging in anything unless it's a mission, but it can be hard and is a big part of the game. But frankly, I have seen far worse games, such as even Call of Duty. It's bad, but not as bad as the average first person shooter.
Alcoholic Reference: There's not really a way to turn this off either, except for doing the same thing as language, turning off the volume. Not even that bad anyway.
Game Difficulty: Assassin's Creed, not just this game, but all soon to be four, are really hard games to get at first. But once you really get into them, (Which I guarantee, you will.) These games really aren't too difficult.