Yes, it's been talked to death. But I asked my students today what they thought characterized YA literature, and they pretty much agreed with Jen & MR in comments, below. Age of protagonist, teen--check. Subject matter, a little edgier, deeper, more emotionally intense than children's lit--check. They also think of YA literature as more realistic, longer, less illustrated, and more complex than children's literature, although we were able to think of many counter-examples for all of those qualities. Finally, they believed that YA literature was less clear-cut, more morally ambiguous, or less structurally "closed" than children's literature. They pointed to Weetzie Bat's rejection of "happily ever after" as a primary example. Again, though, I can think of some children's books--The Giver comes to mind--that are also a little less closed than, say, a fairy tale.
The issue then is, what's the difference between adult and YA literature? Does it really have to boil down to the age of the protagonist? My students were unwilling to call Joyce's Portrait of the Artist as a Young Man, or Dickens's Great Expectations, YA. But they agreed that they fit the criteria they had themselves outlined. They also talked me into listing The Lord of the Flies as YA, though I still resist the label. Again, why?
I love YA literature, and I think I know it when I see it. But at the moment I'm cribbing John Green's characterization of it from a recent blog post: "smart teenagers who talk fast and do stupid shit." That leaves a lot out, I'm sure, but it also brings a lot in, and for the moment it will have to do. (By the way, the whole post--about manic pixie dream girls--is fabulous.)