Apart from its length, what else makes a short story different from a novel?

It was Edgar Allan Poe who offered a definition of the short story which has been widely observed by authors ever since. In a review of Nathaniel Hawthorne's collection of stories and sketches Twice-Told Tales in Graham’s Magazine, May, 1842, Poe wrote: 

A skilful literary artist has constructed a tale. If wise, he had not fashioned his thoughts to accommodate his incidents; but having conceived, with deliberate care, a certain unique or single...

It was Edgar Allan Poe who offered a definition of the short story which has been widely observed by authors ever since. In a review of Nathaniel Hawthorne's collection of stories and sketches Twice-Told Tales in Graham’s Magazine, May, 1842, Poe wrote: 

A skilful literary artist has constructed a tale. If wise, he had not fashioned his thoughts to accommodate his incidents; but having conceived, with deliberate care, a certain unique or single effect to be wrought out, he then invents such incidents--he then combines such events as may best aid him in establishing this preconceived effect. If his very initial sentence tend not to the outbringing of this effect, then he has failed in his first step. In the whole composition there should be no word written, of which the tendency, direct or indirect, is not to the one pre-established design. 

A novel is not intended to produce a single effect because of its greater length than a short story. A short story is intended, as Poe established elsewhere, to be read at a single sitting. A novel is typically divided into chapters, and sometimes even into "books" or "volumes" because it is not intended to be read at a single sitting. Since a novel may be read in many sittings, each chapter can usually produce a separate effect. 

What Poe meant by the word "effect" is the feeling that is left with the reader at the end of the story. Poe maintained that every single word should be designed to produce that single emotional feeling. In some stories, such as those of O. Henry, the effect is usually produced by the ending. But there are many stories in which the effect is produced by the pervasive mood established from the beginning and sustained throughout. Edgar Allan Poe's stories "The Fall of the House of Usher" and "The Masque of the Red Death" might serve as examples of stories in which the single effect is produced by tone, mood, characterization, and setting. 

If a short story is intended to produce a single effect, then it would seem that a very good way in which to critique any short story would be to start by analyzing its single effect, i.e., the feeling the reader is left with at the end.