There is this idea going around that one's viewpoint character has to be the most important person in the story.
While the main character should be important (otherwise why are they the main character?) The idea that they must be the most important person can, I think, keep a writer from seeing all the story possibilities.
Take Tolkien, for example. He makes a point that his characters are just a small part in a larger history, but instead of detracting, the shift in focus makes his stories surprising and unpredictable.
When I first read `The Hobbit' I was shocked when a random side character slays the dragon. "That's not how stories are supposed to go," I grumbled. "If he's not going to do the heroic stuff, why is Bilbo the main character, huh, Tolkien? Answer me that!" Tolkien spent the next few chapters after his false climax showing exactly why Bilbo Baggens is a hero. He gave us an internal struggle that was far more unforgettable than another monster biting the dust.
One of the things that makes Tolkien so great is that his characters don't have to be the center of attention. They carry an endearing sense of humility that I think a number of modern novels lack. The characters struggle and often even fail. Just like us.
Main characters who do the hero stuff are not a bad thing, but sometimes the best moments aren't the obvious ones. Sometimes to find those less obvious moments, the hero has to stand in the shade instead of the limelight.
