What is the twist in the "The Last Leaf" and how does that twist make the story interesting?

You ask two questions about O. Henry’s “The Last Leaf” here. The first is easily answered, while the second depends more on your own opinion.
The first question asks what the “twist” in this story is. This is objectively clear. The twist in the story is that Behrman painted the leaf onto the wall of the building opposite Johnsy’s window and that, by doing so, he died while she survived. This is a twist because we do not have any inkling it will happen. As far as we can tell before the last paragraph of the story, the leaf hangs on by itself and Johnsy is saved by some sort of natural miracle. Instead, the twist happens and we find out Behrman has, in essence, sacrificed himself to save Johnsy.
The second question does not have an objective answer. It asks how much this twist makes the story “interesting,” and “interesting” is something that each person must define for him- or herself. I think the twist makes the story more interesting because it gives it some meaning. Until the last paragraph, neither Johnsy nor Behrman seem very likable or admirable. Johnsy is self-indulgent and silly, giving up on life because she believes she will die when the last leaf falls. Behrman is an unsuccessful artist who has never really tried to paint the masterpiece he always said he will paint. In a sense, both are pathetic, which makes the story somewhat less appealing. With the twist, however, Behrman actually becomes rather heroic. He finally harnesses the talent he always had and uses it for a very noble cause. He does something to save Johnsy without any regard for his own health. This makes him rather admirable and, to me, it makes the story more interesting.