I can remember DNFing two books because they were just so poorly written:
Prison Ship
by Michael Bowers (go look at the cover. I wasn't expecting much, but the book still failed to meet my low expectations) and
Canticle by R.A. Salvatore. I'd heard good things about Salvatore, but that book was atrocious. Written like a kids book, but one where the author is talking down to the kids, in case they are too stupid to follow his story.
One book that I thought was excellent, but still DNFed was
The Nightrunners by Joe R. Lansdale. Lansdale is a terrific and recommended writer. But for me Nightrunners was too bleak and the writing too good at pulling you in to the hopeless world he was describing. I respect the book, though I won't try to read it again.
For the same reason, I won't read
Jack Ketchum's The Girl Next Door. I know Ketchum is an excellent writer and that's why I've avoided this particular book (though I have an e-copy and a paper copy).