That is a good question. I don't read as much as I should, since having been corrupted by fanfic and lost in the world of non-fiction. (Though, my plan is to remedy this starting next month. Yay for more reading again!)
That said, to be honest, it is very rare that I put down a book and walk away. Even the crappy ones I will read to the end. Maybe it's my stubbornness or my need to know how it ends no matter how bad it is.
Now, sometimes I will put a book down and due to time constraints don't pick it back up for a long time. It's rare, but when that happens it's usually because:
1. Thickly written accents. Look, I appreciate that you want to show your characters have accents, But it's hard to read and it breaks the pace for me.
2. Long, long, long passages of description. I honestly don't care about the history of ivy. Seriously.
Those are just two examples. And I read those two books to the end even if it pained me to do so.
But I can see myself putting a book down and never going back if the author romanticized rape, incest, or any kind of child abuse. I'll pass, thanks.
I'll answer the other questions in a bit. If you still want to ask me anything, please do so here: http://moonshayde.livejournal.com/429555.html
Please feel free to also link me to yours -- I'm behind on the LJ fun stuff -- that way I can do the same for you :)