When it comes to book covers everyone has an opinion. Some look at the design, professionals look at the techniques involved, readers look for clues to the story.
Personally, the simpler the better has huge appeal. I dislike peering through swashes of colour for a subtle hint of appeal. 
Which is why the cover of Hookline's A Young Woman's Guide to Carrying On is one of my favourites. And, at book fairs, I have seen women pick it out from among all the other books on our shelves. It's simplicity says tidy. While the bed says something saucy is afoot. Men, however, think it looks like a furniture catalogue.
At this year's London Book Fair, our new novel, Underground, was on full display - it's verdant tones drawing the eye through the trees and toward a large antebellum home. As a young man, a publisher of graphic novels, gave me a lecture on what was wrong with the cover, a 40-something female editor approached and lifted Underground from the shelf. 'I love this cover,' she said. Needless to say, I reminded the young man that the cover was aimed at her demographic, not his.
So when it comes to book covers, beauty is aimed at the intended reader.
For those looking for more information on the subject, a hugely entertaining look at book covers comes from Chip Kidd at a TED lecture: http://www.ted.com/talks/chip_kidd_designing_books_is_no_laughing_matter_ok_it_is.html