Great flick, my boys and I really enjoyed it, almost as much as The Avengers. I wasn't expecting much based on the critics' reviews so maybe that helped.
The other reviews on this site did a great job of detailing the language and action violence so I won't go into detail on that. I just want to note two things I wish I'd seen in those other reviews:
1. The violence is very comparable to (and actually not quite as bad as) what you saw in The Avengers, so if you were okay letting your kids see that, you'll almost certainly be fine with Thor: The Dark World.
2. The previews were worse (BY FAR) than the movie itself. We saw three different previews that included more language, violence, sexual situations and inappropriate humor than the movie did. If I hadn't read the reviews on this site and known the movie didn't have those traits, I would have walked out during the previews. So if your kids are like mine and love super heroes, and you let them watch superhero movies but not other PG-13 type fare, wait until the previews are over before you take them in. Sad that I even need to point this out, but like I said, I wish I'd known about before taking my boys.