Okay, not the WHOLE truth, but I have a 13-year-old sister, and watching her and her friends choose books has been a very enlightening experience. As far as I've seen, there are two major factors, and everything else doesn't seem to matter much.
1. Cover, cover, cover. It's all about a cool cover. How each person deems a cover "cool" is subjective, of course, but it seems that "pretty" covers go over very well for the teen girls. Yes, we may be sick of them as people in the business, but the truth of the matter is that they WIN when it comes to the target audience.
And what's interesting is that the cover often has more sway than other people's opinions! I will say a book is good, and the first question is, "What does the cover look like?" If I show my sister and she likes it—she'll probably read the book. If not? She won't.
2. Word of mouth. If all her friends are reading a book, my sister will usually want to read that book more. She also trusts my opinion as a writer, and my mom's since she is a voracious reader of YA and MG (works in a school, constantly giving me recommendations). Then comes the opinions of some of her favorite authors. But again, still has to have a cool cover.
There's a lot we stress about as writers that doesn't reach our target audience at all. This is both good and bad. Many teens don't check reviews, I think, instead trusting in peers. But they also don't see the majority of online promo. The only time I have seen my sister look up an author is after she read and loved the book, and even then it has only been for her very favorite books, not all of them.
For me, it's a reminder not to stress too much over all the finer details of this publishing thing. My job is to put a great story between the covers, and I have a great publisher to put an awesome, marketable image on the outside. The rest is really up to the readers. AKA: I can't control everything, and that's okay. I'll just keep writing the words behind the cover and hope for the best.