The perspective! The difference between their ages (I really love that Crowley is young and handsome and Aziraphale isn't that much)! Details (so many details especially the hands)! Shading! Pencil! I really really like you can see the pencil lines (I'm not an artist I don't know how to describe them) But what really makes this picture for me are Crowley's sunglasses since they seem to just be flying in the corner and it amuses me greatly.