In most games, as the difficulty of the challenge increases, the experience rewarded for completing it also increases. As players gain more experience points or levels, the amount of experience needed to level increases. Thus, as the player character strengthens from gaining experience, they are encouraged to accept tasks that are commensurate with their abilities in order to advance.
Some RPGs (notably MUDs and MMORPGs) place a limit on how much experience a character can gain from a single challenge, hoping that it will reduce the effectiveness of powerleveling.