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

The correct answer is - A short story normally does not have a complicated plot; a novel can have a complicated plot.

Because of its length, short stories tend to have simple plots that can be resolved within a limit number of pages. On the other hand, novels do not have such constraints - a novel could have thousands of pages, whereas short stories don't have such a luxury. If a short story gets too long, then it becomes a novel - it is no longer a 'short' story.