This story has a breezy, fast-paced feeling despite dense description and a plot about illness and death. The opening paragraph sets the tone, even ending with an exclamation point:
In a little district west of Washington Square the streets have run crazy and broken themselves into small strips called "places." These "places" make strange angles and curves. One street crosses itself a time or two. An artist once discovered a valuable possibility in this street. Suppose a collector with a bill for paints, paper and canvas should, in traversing this route, suddenly meet himself coming back, without a cent having been paid on account!
But underlying the possibly frivolous reasons ("they imported some pewter mugs and a chafing dish or two from Sixth Avenue, and became a 'colony' ") that artists gather in Greenwich Village—home, at that time, of cheap rents—a more serious reality looms. Pneumonia arrives and with it the threat of death. Pneumonia too, however, is described in a breezy, comic, detached manner as "not what you would call a chivalric old gentleman."
The breezy detached feeling continues even as Johnsy contracts pneumonia. O. Henry, perhaps reacting against Victorian death bed scenes, is trying to catch the modern tone of the artists' colony. Johnsy is convinced she will die, and the doctor tells her roommate that Johnsy must have hope in order to live.
As we know, an ironic twist ends the story. There's even a breezy quality to this, as Johnsy's roommate, Sue, tells Johnsy that the old artist Behrman, who lived upstairs, died of pneumonia caught while painting the leaf that kept Johnsy alive: "Ah, darling, it's Behrman's masterpiece—he painted it there the night the last leaf fell."
The feeling of the language is curiously at odds with the seriousness of the subject, forcing the reader to think all the more to absorb the gravity of what has happened.