What makes O'Henry a great story teller?

O'Henry is one of the born story-tellers of the world. He wrote so naturally and without much difficulty as if it was but to breathe to him. He is a master of tragedy, romance and extravaganza, of tales of mystery or of common life/common people, with especial skill in surprise ending. In fact, when one talks about surprise ending, O'Henry rests on the top of the list.

What makes him great in my opinion can be listed as such:

*the variety and colorfulness of his characters and the places.
*unexpected ending.
*tamed, elegant and unexaggerated humor.

What would you say about that?

:yawnb:
I have finished his story "The Cop and the Anthem", and I'm now an admirer of his masterpieces.