With a novel, you have the reader with you a lot longer, and you owe him a lot more. Obviously you have to have a plot - I say "obviously," although I think a lot of fiction doesn't, and nothing seems to happen. But to me, there should be something that happens, and it should be at least vaguely plausible. And because the readers are going to be with these characters for a long time, you have to get to know them and like them and want to know what happens to them.