A lot of factors go into whether you keep reading a book. It might be that you picked up a genre you don't normally try... and realized why you don't usually read it. Or maybe you can't click with the writing; it just sounds too stilted and unnatural. Many many times, it comes down to patience.
How often have you read a review that went along the lines of, "Once you get past the beginning, things start to get going"? I know I've personally had friends say to me, "Yeah, it's kind of boring in the middle, but the ending's awesome." With these books, it seems to boil down to whether or not you're willing to slog on for a hundred or so pages to get to the really good part.
So how patient are you? Would you be willing to take on the task of finishing the book even though it looks as though it'll take effort? Or are you the one with the full-to-bursting "did-not-finish" shelf on Goodreads? And what makes or breaks a book for you?