Fallout 3 is a post-nuclear simulator - and a great one, and the reason it's great is it's realistic: as in if you seriously piss someone off, they're either going to a) cuss you out, b) cuss you out, then hurt you, c) get someone else to cuss out and/or hurt you, or d) get a whole town to do so. The whole point of the game is choices, and there's a lot of them to make, no matter what route you take - you can wipe out whole towns/races if you decide to, or you can protect children and feed the needy.
The ratings set in the What to Watch Out For section is right on track - it's bloody-gory, you take drugs for benefit and can get addicted (the game suggests that addiction can be cured instantly by going to any doctor you find), you run into prostitutes occasionally, and the language can be foul; I'm fairly sure there's no filter available as in other games (Gears of War, etc.).
Having said that, it is one of the best games I've ever played, and I would recommend it to anyone who doesn't have a weak heart (it's 10x scarier than any of the Resident Evil games).