Being the iconoclast (and realist) that I am I suggest that maybe it takes more than a good cover to sell a book, and that there is a point of diminishing returns. By that I mean that a poor quality cover
Of course, this raises the obvious question: How are we defining good? While many give lip service to marketing features like title and representation of content, in practical terms, most are defining it as artistically attractive. Of course, most book cover designers plying their trade on the internet today are not marketing specialists, they are artists. They think like artists and think in terms first and foremost of symmetry, balance, color wheels, composition and design. Nothing wrong with that. However, a well designed graphic on it's own sells nothing. From a marketing perspective, the cover must not only be attractive, but must communicate enough of the content of the book to cause the reader to click through to the sales page. A less pretty book cover that communicates the content of the book is going to generate more click throughs than one that is beautiful, but does not.
Good art does not necessarily translate into good business. Unfortunately, when you say something like that, the critics of indie publishing and even many in the field think you are talking about poor quality covers that are just thrown together. That's not the point at all. The point is that a very create, very beautiful, very expensive cover design is not necessarily a good sales tool if it's only value is beauty.
The Kindle (or Nook or Sony or Smashwords) search engine is not an art gallery. People are not browsing in order to simply see pretty pictures. They are browsing to find something to read. And if it is an ebook, after they purchase it, they will rarely see the cover any larger than about an inch square on their reader. So, what is the value of the cover?
The value is in it's message. A cover that has fewer artistically skilled design components, but which clearly communicates to the reader a reason to buy the book or at least read the sample is going to be more effective than a pretty cover that does none of these.
So, from a marketing perspective what should be on the cover.
1. A simple, but bold graphic. This graphic should not be complicated. Avoid collages. Avoid pictures with too much ghosting. By that I mean a semi-transparent figure in the background. That is a common, romance novel technique that comes from the days of selling print novels in brick and mortar stores. But as a thumbnail, those subtleties are lost.
2. A graphic that communicates the substance of the book. You know what I hate? It's book covers that have two young people on the cover, but when you get into the book, you find the main characters are middle aged. Sometimes a cover can be beautiful, but misleading. Find a graphic that communicates the substance of the book.
3. A clear, descriptive or evocative title. For nonfiction this is easier. You have a topic and you want your title to reflect that. For instance, my next Bible study will be called "Troubled on Every Side: On Being God's People in Difficult Times." The main title will be in large letters. Someone is searching for a Bible study they will see that in big letters. In smaller, but still readable letters will be the explanatory subtitle. Nevertheless, Just "Troubled on Every Side" gives a good solid idea of the general topic to be covered.
For fiction, it is harder. But it can suggest the theme. For instance, Stephen King's epic, The Stand, doesn't tell us everything about the book, but it gives the central idea. These people are going to take a stand. Lillian Jackson Braun's Cat Who mysteries. Always include a clue to the basic plot of the book. The Cat Who Sniffed Glue let's us know that glue will have something to do with the story.
4. Clear Readable Fonts. Some people want to play games with fonts. They are looking for something clever and then the reader ends up trying to decode the title. Remember someone is likely to spend a second or less looking at your book cover, that title needs to stand out.
Yes, you want an attractive design, but if it doesn't sell what's inside, it is pretty useless.