What is the most interesting feature in the story "The Last Leaf"?

The most interesting feature of O. Henry's "The Last Leaf" is the ironic reversal of the story and the manner in which it is achieved.

When Sue first asks Mr. Behrman if he will pose, as well as informing him of Johnsy's "fancy" that she will also die when the last ivy leaf falls outside her window, the little curmudgeon "shout[s] his contempt and derision for such idiotic imaginings." He exclaims that it is foolish to think that one should die just because the leaves all fall from "a confounded vine." Furthermore, he refuses to pose for Sue.

Apparently, Mr. Behrman is an irascible little man. However, he does change his mind after Sue calls him an "old fibbertigibbet," an old-fashioned word which means one who is overly talkative and flighty. Perhaps Behrman resents being called this because he agrees to come upstairs and pose after all. Or, perhaps he lets his veneer of gruffness down; for he then exclaims that their lodging in Greenwich Village is no place for someone as good as Johnsy to lie sick. He exclaims with dramatic irony that he will paint his masterpiece and they will all move away.

When Sue and Behrman reach Johnsy's room, they peer out the window with trepidation. Then they look at each other momentarily without speaking. Behrman quietly poses; later, he returns to his apartment. The next morning Johnsy asks Sue to pull up the shade. "Wearily Sue obey[s]." Surprisingly, the ivy leaf remains there against the window; nevertheless, Johnsy insists that it will fall and she will then die. Yet the next day it is still there. With resolve after this occurrence, Johnsy tells her loving friend,

"I've been a bad girl, Sudie....Something has made that last leaf stay there to show me how wicked I was. It is a sin to want to die. You may bring me a little broth now...."

With Johnsy's will to live revitalized, the doctor gives her "even chances" to recover. But, he informs Sue, he must go downstairs to Mr. Behrman who has contracted pneumonia and is going to be taken to the hospital to be made more comfortable though he has no chance of recovery.
After Johnsy gets stronger, Sue tells her of Behrman's heroic deed:

"...it's Behrman's masterpiece—he painted it there the night that the last leaf fell." 

This moving statement of Sue's provides the surprise ending and adds great poignancy to this ironic reversal in O. Henry's "The Last Leaf."