What Makes a Story into YA?

As I've admitted before, my YA knowledge is limited. But I've been thinking about it a little bit, wondering what it is. What is YA?

Certainly, this new designation has changed things in recent years. YA is IT in the marketplace these days, or so it seems. But what is this IT? Is it merely a matter of marketing? Or is there something more?

I know some of my readers here at The Alchemy of Writing are YA writers, so I'm hoping they'll chime in.

Just what is this genre? Or is it a genre, even, in any recognizable sense? Is it a marketing sping for particular works from a variety of genres, or something coherent in and of itself?

I've read very little YA, or at least very little YA that was YA when it came out. What's Old Yeller? I'm guessing it would be middle grade/YA now, whereas it was maybe just sort of a book when it came out.

I have read Harry Potter, which seems like a middle grade story that eventually becomes YA. It reminds me a little of the Prydain Chronicles, which in the present system also seems like a middle grade that transforms into a YA. If I can even say that, considering I'm not sure what YA is...

I've also read The Hunger Games, and here's where some of my confusion sets in. This is one of the big, hugely popular YA books, one of the vanguards of the form. And yet, having read it, I had trouble trying to determine what about it was YA... with the exception that the characters were young. Though, really, they often didn't even seem young. Whether you slap on the age as 17 or 27, I don't think there would have been much difference. The one exception, perhaps, was the confused and somewhat angsty who-do-I-like love triangle.

The book doesn't really deal with teen life, as it looks like now (or as it looked like in the past). What makes it YA? Is it simply that the characters are young, and the publisher is hoping to tap a youthful market? Do adult books scare teens, while slapping on a YA label makes it seem okay?

I read a blog the other day (I can't remember where), about a YA writer who hadn't read fiction since high-school, because adult fiction was dry. Yet, picking up a few YA books, they found their way back into reading fiction, and then writing it.

So: is YA somehow different? And, if so, what are the mysterious elements that make it different? In terms of the actual stories, what is that has fuelled the boom in YA? (If, of course, there is a YA...)