Chiefly good design- really beautiful art can look muddy when you shrink it down in size, so a good sense of graphic design, and clarity will result in better visuals. Not really my area of expertise, lol. Otherwise, like, obviously certain genre elements, cliches, etc, are going to be more marketable, but vary from genre to genre.
It's fair that certain genre stuff is more popular, so covers that convey that will be more clicked upon. A cover that is "different" needs something grabbing to make up for what it lacks in attractive convention imo.