There are a few things that distinguish "Young Adult" (YA) books from other books. First, the MC is a teenager. The rule here is that kids read up, so 12-year-olds will read about a 16-year-old MC more often than the reverse.
The other distinctions are sex and language. Usually, there's no sex in YA books. If there is, it's not described (e.g., book 4 of Twilight). I've also noticed one other exception to this: if sex in the book is THE BIGGEST MISTAKE OF THE MC'S LIFE. The best example probably is in Cast & Cast's MARKED.
So, there's a not-too-subtle message in the YA lit about sex.
As for language, the line's a little more blurred. Most YA books avoid the hard-core profanity, which for the sake of my readers' delicate sensibilities, I will refer to as the F-word, the S-word, and the C-word. Oh, and the A-hole word.
If you do not know which words I mean, please do NOT email and ask. I'm a lady, and I dare not even type them so far from my fainting couch.
I'm in a grey area in my book MINDER with "God" references in both spoken and internal dialogue. Is it OK for a character to say "Oh my God" when something shocking happens? Is this blasphemy? Will it get my book banned in certain parts of Mississippi? Is that a bad thing?
Interestingly, there's no line on violence. Characters can be decapitated, eviscerated, ex-sanguinated, or otherwise killed and/or mangled in horrible ways in YA fiction, even in books with middle-grade appeal, like Harry Potter.
So, having your MC's BFF sucked dry by a vampire in lurid detail is cool. But don't you DARE let her light up a cigarette.
UNLESS that's the reason the vampire decides to kill her. What can I say? Smoking's a dangerous habit.