Oh, that's the worse. It makes it so hard to finish the book, and when you don't care if good things happen to them, or your like I'm so glad the awful thing happened to you (but not in a cheerful, yeah the evil warlord is dead, instead, gawd thank goodness that's over)...it's not fun.
Historically improbable plot lines and casting in movies and books.
Novels with unlikeable protagonists?