All a book needs to catch my attention is to have a plot, characters, dialogue, and understandable prose. It doesn’t have to be an interesting plot or good dialogue; every book teaches you something, even if it’s just about the author’s fetishes or hangups. (Twilight, I’m looking at you.) I think reading everything that’s out there gives you a well-rounded understanding of books and a good crap radar.
I’ve finished some really terrible books, and some seriously long, meandering doorstops. For some reason, the only thing I could never finish was Naked Lunch. In a book with conventional narrative, my brain just goes on autopilot and I don’t even notice that I’m reading. With Naked Lunch, it was like I had to individually comprehend every word and it took… so… long.