What are the main structural differences between short stories and novels?
Typically, though not always, a short story follows one basic plot-line, which is its exposition, rising action, climax, and resolution. Short stories tend to focus on one main conflict and the storyline progresses in order to resolve it. Novels, on the other hand, can present several conflicts at once. As a result, several plot-lines can progress simultaneously. Anti-climaxes are also more prevalent in novels than short stories.
Because of the general difference in length of a short story to a novel, short stories often lack the character development that is both possible and utilized in a novel. Short stories often have fewer characters as well.
In my opinion, short stories tend to utilize irony as a key piece of an entire story more effectively than novels. It would be difficult to base a lengthy and complicated plot on one key element of irony. Short stories, on the other hand, do this all the time. It seems that because of their shorter length, irony (and ironic humor) is more focused and often plays a more significant role than it does in novels. To paint an analogy, I think of skits (or sketches) as compared to full length dramas or films. Successful skits (think of "Saturday Night Live") tend to be based on one or two elements of ironic humor. When such skits are "extended" into full length movies (think of any "Saturday Night Live" skit that has been turned into a film) the humor often becomes ridiculous and over-the-top, and just as often, the storyline suffers as a result.