e-book covers!

First of all, a “cover” for a Kindle book really isn’t a cover at all.  It’s a flat image that will be displayed on various web pages – most likely as a thumbnail, but also in a larger format if the user wants to have a look.  This means it’s important for the image to work as a thumbnail in order to attract attention.  Another thing that follows from the cover not being an actual physical cover is that there is no spine, and no back cover.
In a bookstore, when a book interests you enough to pick it up, you might glance at the cover, then at the back, then start looking inside.  Therefore, for print books, not only does the front of the cover have to be good, but the back must be designed correctly too, with a catchy quote or blurb that captures the reader’s attention and makes them want to spend more time with the book.  On the web, none of this happens: readers who want to know more click on your book, and an Amazon page pops up with all kinds of information about the book: blurbs, author bio, reviews, and so on.