If you want a quick lesson in what works and what doesn't in terms of the design of e-book covers, take a look at the entries in the December Cover Design Awards at thebookdesigner.com. Designer Joel Friedlander provides a (mostly) constructive critique of each.
You'll find examples of designs that are totally professional and some that make you cringe. The most common faults:
* type that is hard to read (too big, too fancy, obscured by the image)
* images that are confusing (collages, especially, are hard to pull off), or don't match the typography, or are too generic.
* compositions that draw the eye to the wrong place, or make the cover seem crowded, or are made up of elements that fight each other for attention.
Generally you are better off hiring a professional rather than trying to do it yourself. If you do it yourself, get feedback from someone who isn't a friend or relative and knows something about design. Clearly several of the people who entered their covers in this contest failed to do that!