For completionists, collecting all of the Gold Coins in New Super Mario Bros. Wii (there are three in each world) won't simply be a matter of saying "I did it."
The coins act as currency, which players can spend on unlocking "Hint Movies" in Princess Peach's castle in World 1. These "Hint Movies" are playthrough videos of Nintendo's top testers showing off in-game secrets (like "Infinite 1UPS") or some sick, nearly inhuman playing.
Above is one example, the same one shown to us by Shigeru Miyamoto when he visited New York City a few weeks back. It features three players supporting a lazy Luigi through World 1-3. Watch as Luigi himself does nothing, and the players bounce him off enemies and spit him back and forth from beginning to end. Luigi ends up with four additional extra lives by the levels end, as well as picking up most (if not all) of the game's power-ups and secret coins.