When I was younger, I was of the opinion that if I started a book, I was duty bound to finish the book whether I was enjoying it or not. Don't start something you are not going to finish. My grandmother's mantra - from everything to housework to sewing to projects.
However, the older I get the more I realize that there are too many books and not enough time - and time is too valuable to waste on something you aren't enjoying.
Normally, I will give a book three chapters - I figure at the end of that, if I'm not invested in a book's plot or characters, I'm not likely to be. So, there's one reason to stop reading a book. I get to the end of chapter three - if I don't care whether I finish reading it or not, I'll put it down. If the writing is good, I'll probably hang on to it, because I recognize it might be me and not the book. I'll put it aside and try it another time.
However, if the book is poorly edited, it doesn't even get the three chapters. And, unfortunately, that's true even if I'm interested in the story, because it drives me nuts. An occasional typo- okay, I can live with that. But a book that has obviously not seen an editor's pen... sorry. I can't do it. I spend all my time mentally editing it and when I read, I want to take off that hat.
Another thing that will make me put a book down is blatant head hopping (yes, I'm speaking to you, Nora). Drives me up a wall. Funnily enough, I love JD Robb's writing.. go figure.
I'm normally very forgiving in my reading though...different strokes and all that. What about you? What makes you not want to finish a book?