I've been giving this subject a lot of thought lately.
As I go through the "wish lists" of agents, where they state in their profiles what exactly they are looking for, the amount of agents looking for YA fiction is pretty overwhelming. I suppose this could probably be attributed to the Harry Potter and Twilight crazes, but if that's what they're looking for, I'd guess that is what is selling. I'm sure the agents have their finger on the pulse of the market. So, I'm leaning toward marketing the current book I'm writing as YA fiction.
While I'm writing it, there have been things I've had to do to not cross over into commercial fiction, but as a larger concept, I think there is a pretty fine line between YA and "adult" fiction anymore. Perhaps the vocabulary is a bit understated in YA fiction, and maybe there are A FEW subjects that aren't broached...but even in those categories, I think the current generation of YA readers needs to be given more credit.
It's not that vampires and wizards aren't entertaining, but what is to say that a book can't be YA fiction if it covers pregnancy, marriage, sex, drugs, alcohol or anything of the sort? I know a lot has changed since I was a teenager, but it seems that these days, people in that age group probably know more about those things than even their parents do. Albeit, we all thought we did at that point in our lives, but for the first time ever, it might actually be true.
I've been shocked lately as I've watched episodes of True Life and Teen Mom on MTV as I watch what these "teenagers" have to endure before they are even old enough to take a sip of alcohol. Many of them have children, have been in rehab and have endured things some people in their 50's and 60's know nothing about. So, perhaps, they COULD relate to more mature subjects and we just aren't feeding them what they need. I don't see many YA books that address those issues, even on a fictional level.
Perhaps at this point, the only subjects that would be "off limits" in YA books are those which they might not find interesting, such as college being as they haven't reached that point in their lives, or politics since it's unusual to be into politics before one can actually vote, but as for the rest, I think they should have the option to read stories about the things that are affecting their lives the same way that adults do.
I think that genre is undergoing an evolution...or at least its readers are, which makes it an exciting time to be a writer, especially one who is looking to enter that particular arena.