A cover can make or break a book. What makes that 'right' cover though? I recently had a time with one of my authors, the cover artist for her book, and the proposed cover that--well it was a good cover. It just wasn't the 'right' cover for that book. I knew it, the author knew it, unfortunately the artist had a hard time accepting it. But now a new sketch (just the rough outline) by a new artist has been present to the author who nearly choked with joy when she called me to see if I'd seen it yet (hadn't, had been out for the evening). What made it right? I couldn't tell you. I know when I see it. (I've seen it now) She (the author) knew when she saw it. There were no words to adequately explain why it was right, but you just know it when you see it.
It's not even to say it's a matter of opinion. Others who were impartial to a degree anyway, were asked and they'd agreed on the first cover's overall decent quality but not quite right for the book in question... so what makes that? Perhaps it has more to do with tone than it does with actual elements. It has to match the style, tone, and feel of the book.
It has to convey a feeling for what you are going to experience. Sometimes even a black cover with a stark symbol on it will do it. Sometimes a bodice-ripping clutch... whatever the book has inside must somehow be conveyed in a picture on the outside. They say a picture is worth a thousand words, and in this case it just about has to be... and when it's right--it's really right.