Experience Rewards in RPG's (part 1)

The traditional model for RPG-type games is to reward “experience points” for killing an enemy. In video games, it is common to see a system designed to present increasingly difficult enemies that are focused on killing the player's character. If the player defeats one of these enemies, a certain number of experience points are rewarded to the player's character for that kill. Once a pre-determined number of experience points are earned (usually this is represented onscreen by a progress bar or number,) the character in the game “levels up,” earning new rewards and abilities. This design works very well, and the carrot-on-a-stick approach of these rewards encourages the player to keep playing, slaying hordes of enemies to level up and make his character more powerful, usually so it is easier to slay hordes of enemies. Of course, as the character progresses through the game, the enemies become tougher to match the character's level, so it is a perpetual drive forward and challenge until the game is solved. However, is this formula the only way?