I'll be the first to admit that I don't know anything at all about how to make an outstanding book cover. I know one when I see it but how to get it? That's the value of art departments. I think magic is involved somewhere along the way too.
There are good covers; there are amazing covers; there are godawful covers. Then there are EFFECTIVE covers: covers that make you reach for the book.
Here's an article about the various covers considered for THE SUMMER OF DEAD TOYS by Antonio Hill. All of them are gorgeous. The question then becomes: which one is most effective.
Do you agree with the choice they made?