Justine Larbalestier over at her LJ asked a similar question the other day, actually in relation to killing off animals and whether that puts readers off...and if not, then what does?
I actually wrote the post on killing a cat to see if it would get a reaction. Nada. Maybe people were just too polite to tell me what they really thought. Or maybe they looked at the title and didn't read it! (It was absolutely true, I hasten to add. And I deliberately didn't mention that I am a cat person who is immeasurably distressed by stray cats in trouble, etc. However, I am also a very pragmatic farmer's daughter, so accidently killing a cat that had unhappily chosen to sleep in the engine of my car was, to me, more messy than traumatic.)
But back to the topic of this post. What stops you in your tracks when reading? I mean what actual subject matter, rather than poor writing and stale imagery, poor characterisation, overwriting, incorrect facts - all things which put me off every time.
Do sex scenes bore you to tears, as they do me (mostly)? Or is it a torture scene that will make you chuck the book across the room? Too much graphic violence? Killing off a child? Weepy deadbed scenes? Dream sequences that you the reader think are real only to have the character wake up? (Geez, I hate that one.)