What do I enjoy? Something that so totally suspends my ability to disbelieve that I feel for the characters. Make me laugh or cry, feel fury, or compassion. If I am in that place, I am enjoying myself and I know I have a good book. Make me care and you have made me a fan.
Devoid of these excellent attributes, then I will start to glaze over and probably not finish the book. An author who cares about his or her work will be experiencing these emotions as they write. This comes through, loud and clear.