So what makes a book YA? Most of it seems to be due to the protagonist. A YA protagonist is 12-19 years old and the whole story starts and ends in that same, teenaged POV. Subject matter and themes seem to also skew a story into YA. Most are about growing up, coming of age, experiencing things for the first time. That 'first time' feeling is something you really want to capture in a YA novel. I don't know if I have a handle on that in my new novel or not but it IS something I did fairly well in Angel Odyssey...which is another reason at least two agents think it should be YA instead of Epic Fantasy.
Back to subject matter, it used to be, YA novels had content limitations. No explicit sex or violence or bad language. Now some of that is still in effect, if you want to sell to Scholastic or want your book in school libraries, then you have to get past the gatekeepers (Teachers, librarians, parents). However, the YA market has changed a lot in the past ten years. You have stories with rape, murder, kinky sex...all the stuff of adult novels.
Now, that doesn't mean that a YA novel is just a normal genre novel with younger characters. A lot of writers, God-willing not including me, don't get the voice of their characters right. A teenaged character needs to sound and feel appropriate to their age. They shouldn't use dialog that a 40 year old uses. That doesn't mean putting in pages of "uh" and "I don't know", we don't need verbatim teenaged dialog. But there is an energy and a rhythm to the conversations young adults have, we need to try to capture that. That means parents with kids in that target age have a leg up on writers like myself. But there are ways to get some authenticity.
School visits, author readings at libraries, even volunteering at Boys and Girl's clubs can be a great way to get close to that young adult sound, to see the emotions and dramas that we've mostly healed over by now. Talk with other YA writers, read good YA books (or even movies. Tangled does a good job of capturing what a teenaged girl is like, at least according to former teenaged girl, my wife).
One thing YA novels definitely need is a satisfying ending. That's probably true of just about every type of non-literary novel but it seems particularly true with YA novels. A good ending has:
-An echo of the themes of the story. This is where you wrap everything up. And if you can echo your first chapter, where you set these themes in motion, all the better. If your last line echos your first line, that's cream on top.
-A display of feelings and emotions. A YA novel is all about emotions, make sure those are on display but like they say, "if your character's cry, your reader won't."
-You need to show the consequences of the protagonist's choices. The good and the bad, both need to be on display as well.
-Possibly, a discussion of what the future holds for the characters. Not necessarily an Animal House where-are-they-now montage, in fact DON'T do an Animal House montage. Ever. And if you're writing a series, you may not want this either but for standalone novels, yeah, hint at where things are going.
I wish I knew more but I'm still exploring, learning. Humor is good. Romance is damn near essential. The rest, well, I'll keep you informed. But I'm really enjoying what I'm writing and I think I would have liked reading it back when I was that age. Ultimately, that's what matters, I think.