One of them is characters. You have your main characters who plug along on the quest. And you have your bit character whom your main ones bump into and then leave behind. Compare Frodo's plot with that of the rest of the Fellowship, who were no longer on a quest as soon as the Fellowship broke, and so could keep on bumping into minor characters, or even major ones, over and over again. Thus developing a web of social interaction.
Depends on the story whether minor/major characters will work, but the lack of them has its fun aspect. Foils, for instance, can only flare once, and not develop the contrast; if you need more contrast, you need more characters. And subplots have to revolve about the main characters. Even if you promote a bit character to point-of-view and followed his adventures before and after, it's hard to tie into the main plot. This means that if the story needs to be expanded in length, it can be quite interesting to make it more complex and not just longer.
Writing is full of complications.
