1. Unlikeable main character. I know, the author has good reasons. The character is supposed to fill some kind of archetypal role that necessitates him stealing, being accessory to murder, stabbing a friend in the back (literally or figuratively), etc. Maybe she's a complainer or gossiper, and the point of the story is her redemption. I can't find room in my heart for the first reason, although maybe I can understand the second. I like redemption features in characters who aren't the main character. But if I'm going to follow someone around in a book for 200+ pages, I need to like them. Yes, Crime and Punishment is a classic. No, I actually don't read it over and over as a bedtime story.
2. Too much psychic distance--ie, writing about a character instead of seeing through his/her eyes. This is the root of most books I find boring. I don't care so much about what a character looks like as what their moral issues are and the way they see the world. This distance problem happens within a variety of issues: too much headhopping between different characters (common in adult books but not kidlit), not enough internal reactions from the character, using the character to explain the worldbuilding/plot setup while forgetting the character's humanness (sort of, As you know, Bob), and writing down to the reader (usually when lofty adult writers decide to spin one out for the kiddies, who are of course, dumber. Grr!) This is a key difference between kids' books and adult, so if you are an adult writer who honestly has a yen for writing for a younger audience, you would do well to study this difference.
3. Unnecessary cruelty to your characters. I don't mean putting them through the wringer to force them to make hard choices and grow. Voldemort kills many people Harry loves, he tricks Harry into leading people Harry cares about to death, and in the end, Harry's got to make the ultimate sacrifice to defeat Voldemort. I'm not talking about that. Readers LOVE seeing characters pressed to a wall, because they anticipate a major payoff/triumph commensurate with the sacrifice the MC is giving. No, I mean adding a situation that does nothing for the plot or character growth, that seems to just add shock value for the reader, and that, while huge and horrible, doesn't get more than a meh, that stinks, move on to the next plot point from the author. Like hm, having a book about a runner who has to retrain after an accident, but dumping in a scene with a pedophile and then just moving on without really addressing how it affects the character more than, eh, some people out there are just bad. Okay, let's get back to training. That sort of thing a) makes it seem like the author approves of pedophilia and b) is completely nonsensical within the context of the book. It's not something I see often in books, but I have seen it more than once, and it definitely promotes me throwing said books against the wall.
4. Nothing happens whatsoever, ie, no plot. My sister, who is a poet, has a lot higher tolerance for pretty writing at the expense of story, but the main point of a story to me is...a story. Without a story, I might as well go back to washing dishes and polishing doorknobs. The multiple plot threads where the characters almost interact but don't...yeah, another wall-banger. I want strong writing--I LOVE strong writing--but I want it with a story.
5. The book that skips the climax. Seriously. I have read a few of these. Building up to something big and then skipping it altogether is worse than having it all be a dream.
So to turn this right-ways around, I guess what I want to read about is a likable character I can share thoughts and feelings with, who goes through real challenges that get enough respect from the author to be addressed with gravity commensurate to the situation. Ie, I want a book that has heart. And those are the books I love to name and talk about!