Difference #4: Subplots
Usually the short story can’t support a subplot. If there is too much going on, the reader will find it hard going. The piece generally concerns one basic storyline, one or two themes at the most and no more than two or three characters.
In a novel, subplots give the story a new dimension. They provide layers, texture, complications, back story and crank up the tension. But at the same time they need to run parallel and drive the main conflict.
Introducing new characters deepens the story and keeps the reader interested. Also, they show hidden impulses behind actions of the major characters. But they must happen for a reason and move the story forward. Also be sure to connect and relate them to your main plot. Eventually they have to be resolved by the end of the story.