Salon has an article about the audio version of Code Name: Verity by Elizabeth Wein. In it author Laura Miller says, "Although published as YA (Young Adult) fiction, it’s a bit of an odd duck in the genre. Its accounts of Nazi torture and death camps are serious and frank (and, it should be added, quite true to the fates of the many brave young people, male and female, who fought for the French Resistance), which will make it too disturbing for some youthful readers. And there’s no dreamy romance, an apparent requirement in YA books for girls these days."
Now, I think an argument could be made that Verity could have been published as adult because the main characters aren't teenagers and aren't living a typical teenage experience. But because the accounts of torture and death camps are too disturbing for youthful readers? Are torture and death camps that different from all the other disturbing material that YA covers--rape, death from disease and accident, and mental illness, for instance?
Miller's comment about the romance requirement--I have heard rumbling about that, but never that the absence of that element is the mark of an adult book.