Designer Jessica Bell rounds up the major elements of strong cover design in her book Can You Make the Title Bigga?
Use open space.
Choose fewer colors.
Make colors complement one another.
Jane Friedman's blog features a post from Bell with visuals to help you see the difference these elements can make. Bell's book will give you a starting point when discussing a cover for your book. Or talking about how to improve your covers to boost sales.
Bell says, "It’s important not to clutter a cover. If there are too many elements fighting for attention, there is no focal point, and therefore nothing to attract the eye."
A book cover with space allows the imagery and text to breathe. Utilizing space wisely draws attention to the elements that you want potential readers to focus on.