I was part of an online discussion of writers, recently, where we discussed what types of covers sell best. Most of the people in the discussion were romance and/or erotica writers, so there was a strong bias toward showing lots of skin on the cover. Personally, I loathe headless torsos. A headless, naked male torso, or two, is the surest way for me to snub a book.
There are many factors that go into the process of a reader selecting a book to read, the most predominate is author name recognition. Other factors are title, cover art, blurb by a known name, reviews, and recommendations from friends.
I think a strong blurb does the trick, but in order to get the reader to read the blurb, one needs an eye-catching cover. Of course, what is eye-catching to one person is a turnoff to another. Different people have different preferences regarding sexy vs. tame, headless vs. visible faces, iconic vs. character-based, painted vs. photo, what types of models they find attractive, etc. If the cover art has a subject matter that interests them (something as random as a ship or a tank), it may also catch their eye even if the overall cover is distasteful.
But the less subjective factors that will consistently and reliably catch the human imagination are strong composition, good use of lighting, contrast, and color harmony. Good use of visual storytelling helps, and something unique will also get noticed.
From my perspective, the most important factor about a cover is that it accurately depicts the nature of the story. The goal is not to sell the book everyone, but only the people who will want to read it. Thus, a cover should be honest—in mood, tone, atmosphere, and subject—in matching the writing and story.