I don't like to give up on a book, but it sometimes happens.
I think some books are oversold on their cover blurb (I'm sure that's not the proper term), and end up being either badly written or boring. I will usually finish well written but boring, haha! Also if the main character is not well drawn out, or has qualities that don't make them attractive I won't finish.
Eat, Pray, Love.... fairly well written, but I found the main character so insipid and shallow that I gave up half way through her India trip. Somehow I enjoyed the movie.
Paperback mysteries with cute titles involving a cat or bakery ... I picked one up from a free bookshelf on vacation and could only read a few pages. I know these sell like Harlequin romances, but yikes, what drivel.