Someone somewhere said that YA books provide a map of sort for teens looking for answers. The characters -- whether in fantasy worlds or far, distant planets or on the streets of Brooklyn -- encounter problems and issues common to all teens. Readers are looking for ways to negotiate bickering friends and squabbling parents and temptations galore. Teens hover between the worlds of wanting to fit in and belong and wanting to be different and stand out. They face loneliness and fear, peer pressure and rage, love and lust, and all the feelings in between. Well-written books provide a variety of answers to the questions, with a variety of possible consequences to the choices made. There's a fine line between preaching and exploring -- and it takes a deft author to weave the engaging, thought-provoking series of choices and mistakes, actions and reactions.
The same person also pointed out that teens typically haven't read the myriad of books that "came before" -- the ones that set the tone or the bar or the stage. They're fresh eyes, looking for answers that have already been answered elsewhere. But you, the young adult writer, have a mandate to provide the same variety of answers with the same clever skill as those that came before -- only different. Same, because the questions are the same. Different, because agents have read all the books that came before. Or something like that.