I've been trying to figure out what to say on this subject. I am about as far from an expert as you can get, but they are tremendously important. You have probably at most a few seconds to tempt someone on Amazon, Smashwords or B&N to take a closer look. All that will do this is your title and cover.
What makes a good ebook cover? I think first you have to realize, and this took a while for me, that an ebook cover needs very different elements than a DTB cover at least in large part because people mainly see it in thumbnail size. This means that small details and small or elaborate fonts get lost, often completely. I started out trying to judge my covers by successful DTB covers before I realized what a bad idea that is.
I recently saw someone complaining that no one knew that his mystery was a mystery. He was told that his cover didn't look like a mystery with it's semi-nude woman on the cover. His response was, but it says it is a mystery. The second problem (other than the semi-nude) was that you couldn't read the small font that said mystery.
Remember, in a book store someone may pick up a book and examine the cover not to mention that a mystery is in the mystery section. This doesn't happen that much online. The potential reader may magnify it, but that is highly unlikely if it doesn't catch their attention in the first place.
If you make your cover look like it is from a different genre, then why would someone looking for a mystery bother to look at it? They won't.
My dragon-looking creature on my Wings of Evil cover is a bit of a cliché for fantasy covers, but anyone who looks at it darn well knows it's a fantasy. I hope the interesting artwork (I hope it is) and the bright colors help get past the fact that dragons on covers have been done to the point of cliché-dom. People haven't gotten tired of handsome hunks and beautiful women on the covers of romances yet, so I think I may be right there.
Am I sure of it? To tell you the truth, no, but generally it has received a good reception and positive comments.
Fonts are another problem. You have to be able to read it in thumbnail which means it has to be large for both the title and the author name. Fancy fonts seem to lose most of their detail and even look a bit amateurish in thumbnail size. That doesn't mean you have to stick to TNR. What is too fancy and what is just a touch of flair is open to debate, but at least you need to be sure that it can be read easily when it's small.
That doesn't touch on good design elements. I'm not going to discuss that because it is just plain open to too much debate. What I consider good design and what someone else does may well differ wildly. But it needs in some way to be interesting. Now is that vague, or what?
Please comment. What do you think makes a good ebook cover?