1. Underdeveloped lead characters
This is my biggest pet peeve in YA fiction. It happens all the time. We have some protagonist who is described as having one or two personality traits (stubborn, passionate!) and a unique physical characteristic (something like red hair or violet eyes) and that's literally the extent of this character's development. It really chaps my rear end. A couple key offenders: Claudia in Evernight and Zoey in the House of Night series. Truly cannot abide. In contrast, for examples of truly well developed characters, look at Rose or Sydney in Vampire Academy/Bloodlines or Seraphina.
2. Underdeveloped romance
This is related to the last one. I love a pair of star-crossed lovers. I am not ashamed to admit it. Rose and Dimitri, Will and Lyra, Merit and Ethan...mmmm. But the author has to convince me! And that takes time. Nothing grinds my gears more than than this type of scene (which happens ALL THE TIME in YA books): I walk into the room and a boy is there. Oh, my God. His eyes. His smile. Then, as if it couldn't get any better, he opens his mouth. Oh, my God, his voice. MARRY ME, BOY. Ok, maybe that's an exaggeration, but this really does happen constantly and it is so disappointing. Some key offenders: Meghan and Ash in the Iron Fey trilogy (which I loved very much otherwise!), Mary and Travis in The Forest of Hands and Teeth, Nora & Patch in Hush, Hush.
3. Unconvincing/annoying love triangle
This is basically the same as #2. I love a good love triangle, but it takes work. I need to be invested in the characters and I need to believe that the member of the love triangle who has to decide between the two other members is actually invested as well.
4. Lame, incomplete, illogical, rushed endings
Nothing is more disappointing than enjoying a book only to reach the end and it feels like the author has just given up or was in a huge rush to meet a deadline so s/he's just slapped some stuff together to quickly wrap it up. Off the top of my head, the worst offenders that come to mind are The Reckoning (final book in Kelley Armstrong's Darkest Powers series), and The Girl of Fire and Thorns. I actually really enjoyed the Darkest Powers series up until The Reckoning and The Girl of Fire and Thorns up until the last couple chapters. It's not even that I was angry at how the authors decided to end the books; I just felt like the endings were sloppily done and not at the same caliber as the rest of the book. Humph.
5. Inconsistent/indistinct voice
This is linked to #1. I find that this problem is most severe in books where two lead characters take turns narrating. An example of a book in which it was really well done is Simone Elkeles's Perfect Chemistry. Alex and Brittany each have a distinct voice, and it really makes the split narrative structure work. Code Name Verity is another success. This might be an unpopular opinion, but I would list Maggie Stiefvater's Shiver as a failure here.
I know this is Top Ten Tuesday and not Top Five Tuesday but I have to run! Maybe I will update with another five later in the day. Thanks to The Broke and the Bookish for hosting this weekly event!
What are your turnoffs?