For example, what constitutes a "good" book cover for a bookseller? Newton notes that it is something that pretty much "sells" its similarities with certain other "known sellers." But I believe there is more to this than just that. If I were looking for a steampunk-style novel to read, the cover aesthetics would differ than if I were to be looking for an epic fantasy one. Placing elements associated with one (sub)genre on a cover for a different style of story doubtless would be a recipe for disaster. It would be akin to putting Fabio (not Fábio, mind you) on the cover of an Ursula Le Guin novel. The steams would cross and like Egon said in the original Ghostbusters, that would be very bad.
I personally don't pay too much attention to cover art. I notice if the artist has rendered images well, but as a consumer, I am persuaded to some extent by the type of art employed. I like plain but well-bound books over colorful images. Some of my favorite covers are the ones that Aio did for the American editions of Zoran Živković, such as the one below: