The unique eye. Having a good eye is 60% inherent, 40% learned. The learned part is the studying of and knowing of your equipment. The art is your unique ability to see your subject. It is what others will come to know as your distinct style. As photographer/teacher Les Saucier so aptly put it:
Photography is art and craft.
The craft is the doing.
The art is the seeing and feeling.
It is the seeing and feeling that makes the difference.