it takes a lot to close a book early...
I stopped re-reading Fireflies by Piers Anthony because after a hundred pages I felt plenty well-studied on why I hate the fucking question mark and exclamation point, when they are over-used.
I stopped reading a Ketchum book's bonus novella when it was clear it was about a teenage psychopath" gifting" her drugged twelve year-old sister to her even more messed-up boyfriend.
I stopped reading Angels and Demons when I just couldn't make myself go on without re-reading individual lines and trying to figure out what Brown was thinking, or if there was some greater whole (my opinion only, but something like taking the fibers from cowshit to weave a tapestry) that I was missing, and i realized I wasn't reading a story anymore anyway, I was just dissecting sentences.
Note in Ketchum's case in particular this isn't a "He's a bad, bad man for writing that!" thing, and that battle cry really bothers me, but I have kids around that age, and this world is plenty disturbing at times.....it hit close enough to home I chose not to read it. That's all.
all that said, i usually read to the end of anything I start, although most of the things listed by other posters will determine if I buy another book by the same author later or not.