What makes you stop reading a book? Language? Sex? Violence?
For me, it's stupidity.
I hate stupid plots. Even worse are stupid characters. Especially stupid girls! When I find myself inwardly screaming at the main character in frustration because she can't see what's right in front of her face, it's time for me to put that book down. Or, when the dialogue between characters is so inane and poorly written that I find myself gritting my teeth, I know I won't be finishing that particular book.
I also hate bad writing. If an author's voice isn't authentic and doesn't ring true, or if they try to be too clever in replicating some imaginary dialect...forget it. I can't go there.
Everyone has a breaking point and stupidity in a book is definitely mine. The last Mary Higgins Clark book I ever read was the one where the main character went to an abandoned warehouse in the middle of the night, in search of a killer, without telling anyone where she was going; I thought she deserved to get buried alive. I haven't read Clark since. I guess I have a low tolerance for dumb.
What about you? What makes you stop reading a book?