Where would be the creepiness "kill-point" of the protagonist where you would no longer want to read a book, assuming you had no prior knowledge of its quality? Would a partially human chimera be too creepy? An alien parasite? A chitinous insect humanoid? At what point does a character become too visually disgusting for you to be likely to start reading the book? I personally don't have much of a stake in this question, but someone I know does.