I've reviewed a lot of my lowest ratings on GoodReads, and found a few patterns in books I've intensely disliked. The most common factor, so far, is unlikable characters.
It's not that everyone in the book has to be someone I'd want to be my best friend. And they really shouldn't be flawless and perfect, because that makes for an annoying protagonist. I do, however, need to be able to relate to someone within the text.
The worst character development sin is building cliches or stereotypes. Characters can certainly conform to stereotypes, but they also need to step outside the cliches. Personally, I don't know any walking stereotypes. I want any book world to be populated by the same variety of types I see outside fiction. If not, there better be a strong in-world reason why people are so two-dimensional.
The second sin of character development is trying to make me like characters by telling me I should. I'm sure you've all read those books, where all the other characters swoon in the protagonist's general direction, talking up that character's good points. Then, the text fails to live up to that dialogue. Or, there's a paragraph or two of exposition about why I should like the protagonist because of all the wonderful things he or she did, but then I don't see within the story a character capable of performing all those wonderful feats.
The third sin is the "too stupid to live" trope, sometimes abbreviated TSTL. This is the character who hasn't the common sense to remember to look both ways before crossing the street, and you wonder how they made it this far without getting run over by a bus. I like the lovable dumb ones, in certain pieces of fiction, but when they consistently make terrible decisions, or worse, if the plot hinges on this character's stupidity, I'm apt to throw the book across the room to get it away from me.
Inconsistent characterization, selfishness, and overly relying on alcohol as an excuse for people to sit around spouting off dialogue also bother me. The presence of an alpha male also is another pet peeve, but that's purely personal preference and has nothing to do with the quality of a work. Generally, if there's an alpha male, some strong female character is going to be reduced to a simpering stereotype of femininity, and that irks me. I'm in the minority on that point, though; there's an entire romance genre that proves it.
I've read a lot of books with unlikable protagonists that I've enjoyed, if only because there was someone in the text I related to. If I can manage that much, I'll usually pretend that character is the real protagonist. As you can imagine, literary fiction where the climax of the story is that everyone sucks equally isn't my cup of tea.
How about you? Can you like a book with unlikable characters? What's the last book you've enjoyed that was populated with characters you couldn't relate to?