L.A. Noire is a game for mature players. Rockstar Games has dived into the dark world of crime in the 40's and 50's, and it isn't pretty. That said, it's one of the best games I've ever played. You play an investigator in the L.A.P.D., and much of the game consists of examining crime scenes and interviewing suspects (one of the best dialogue systems I've seen in a game).
Violence: moderate to strong. You'll do a lot of running around and shooting from a third person perspective, but it's not really the focus of the game. You'll see some blood, and the most noticeable aspects are a rather realistic effect that approximates the appearance of a bullet wound on characters who have been shot, and the violence done to the bodies that you examine as part of the investigations (one scene features a family that has been burned alive in their home).
Language: strong. You'd be hard-pressed to find a developer that puts more language in their games than Rockstar. They never shy away from portraying the dark side of things. That said, it's interesting to note *who* uses the language. Most games with a lot of foul language assume everyone uses it, but in this game, it's much more realistic. It's not in some characters nature to curse, so they don't. But if someone does, you can assume they know every expletive in the book as well as some creative derivations.
Sex: Strong. There is a lot of innuendo here as well as strait up discussion of the topic, and several cases relate to sexual crimes of one kind or another. Visually, there are several cases involving women found murdered without any clothing. The nudity is presented maturely, and the characters are not sexualized in any way. It's your job to deal with it, and that's what you do.
Drugs and Drinking: Strong. A major plot line involves drugs, as well as a number of other cases. Most characters drink and/or smoke. It's all here.
Despite all of the mature themes, and at times because of them, Rockstar is able to tell a truly engaging noire story. As soon as I launched the game, I was immediately hooked by the mood. Few games have engaged me so much, and handled such mature themes so well. Too often, games are labelled "mature" for their juvenile and inappropriate depictions of these things. L.A. Noire takes those themes and presents them just as you'd expect a professional detective to deal with them--and because of that, Rockstar was able to craft one of the most believable stories I've ever found in a game. For anyone mature enough to handle the admittedly very mature topics in this game, it is absolutely worth it.