I think keeping it simple is always the best route. Mostly when you design a book cover you want something eye catching that sets it apart from the rest of the shelf but at the same time is true to the work inside. It's really easy to go overboard designing especially when you're trying to cram in all the details. I find myself starting with something overcrowded then just deleting and deleting until I get it right. I think color is also important in designing covers the smaller the pallet the more striking the image. At least that's how I see it.