I'm sure it's quite blatant what the difference is. I simply cannot be happy about a book cover with the character's face on it. WHY do they do this? Books are about using our imaginations, about escaping to a place where we want to be at that moment. And I truly believe that the very same book can create a different escape for each person that reads it. Even though we are reading the same story, we fill in our own blanks in order to make it ideal to us.
But when a face is plastered across the front of the book, the face of the character of which the entire story is centered, our imaginations are instantly limited. The cover tells us: This is how the protaganist looks. End of story. It's so frustrating to me because the picture never resembles what I might imagine in my head. But once I've seen the picture, it's very difficult to create my own ideal world as well as I would have.
I can only imagine that this has to be frustrating to the authors as well. To put your heart and soul into creating a character that you love, only to have a cover artist define that character in two dimensions.