Conflict of Good and Evil
The forces of good and evil exist in a balance, swinging back and forth like a cosmic pendulum. One may appear dominant for a time--such as when good predominated during the Age of Might or evil prevails during the Age of Despair. But if this occurs, the great pendulum will swing once more.
Even the humblest people of Krynn are unwitting participants in this never-ending struggle, with individuals selected to champion the cause of the deities. The characters in a Dragonlance campaign will have to choose sides (for even not choosing is a choice in itself). But men and women are not simple puppets dancing on the gods' strings, and even the greatest heroes possess flaws and weaknesses, just as the most corrupt villains may possess redeeming qualities.