One of the most common gaming conventions is the power-up, an item that suddenly ratchets up your strength or abilities. An early example is the Pac-Man "power pellet," which let you turn the tables on those pesky ghosts, if only for a few seconds. Every power-up, from the invisibility shield in Halo 3 to the agility orbs in Crackdown, is a descendant of that venerable pellet.

Power-ups are enormously satisfying because they allow you to cast off limitations. It's a rush to suddenly be stronger, faster, healthier, or deadlier. And even the slightest tweak to the rules can have a profound effect on gameplay. Toru Iwatani, the creator of Pac-Man, just updated the arcade classic as Pac-Man Championship Edition for Xbox 360. Iwatani reimagined how power pellets work, allowing them to be strung together for increasingly higher scores. Casual observers would be hard-pressed to spot the difference. But for hard-core players, the new pellets represent a subtle, elegant shift in how the game is played.