Let's get a few things out of the way. Next to Arkham Aslyum, this is perhaps the best game of 2009 (Uncharted 2? Not impressive considering the MGS series).
That said, this is indeed a "mature" game, and I honestly believe that this game handles "mature" themes in just that sense, as opposed to the sensational spectacles of games like GTA.
Addressing the "questionable" material, I think that with good parental supervision, teens (not tweens) can handle this game.
With the exception of a rare special decapitation animation during fighting, the violence in this game could pass for a T rated game. There is plenty of blood, but nothing silly or outrageous like the rivers seen in anime movies.
Correcting the Common Sense review, I remember encountering two Desire Demons, who wear something akin to pasties over their nipples. They are succubi, so explaining to your child what a succubi is (and the corruption/sin associated with such sexuality), this should not be alarming to you. It is *supposed* to be arousing (otherwise, they would be sorry excuses for succubi). Those are good teaching moments in the game. There is also another kind of demon in the game which is topless, but it is so grotesque, I doubt it will be a problem.
Yes, there are romances in the game, and the sex scenes have the characters in their underwear (btw, every character in the game has a fine-chiseled bodies like Barbies or The Sims characters. There are no flat-chested women in the game and they all have Jessica Biel-style butts). If Twilight is acceptable in your house, none of this will bother you.
However, as has been pointed out, there is potential for homosexual relationships. With the female character involved, it is at least portrayed as awkward at first. With the male character, it is more than a bit suggestive. Tracking your child's progress in the game, you can see the "relationship" bar to see how close they are to potentially consummating a relationship, should they wish.
So if you've had a talk with your child about sex (if you haven't, someone else will), and they've seen at least one R-rated movie on television (where the commercials cut out the extreme stuff), this game actually is not that bad. I've been gaming for 24 years and I can assure you that there is much, much, worse out there.
If I had to pick one game with "mature" themes to expose my child to, this would be one of them. Just imagine a slightly gritter version of Lord of the Rings, and you've got Dragon Age.