Finished New Super Mario Bros. Wii? Defeated Bowser and collected every gold coin? Pah. That's merely a "decent" achievement, says Nintendo.
You're no expert until you can finish level 1-1 in 44 seconds, or get 20 extra lives from a single fireball spat from Yoshi. Those and several more feats listed on a New Super Mario Bros. Wii Challenges website.
Nintendo has set a bunch of challenges from time trials to technique and multiplayer challenges, and divided them up into three difficulties. There's some cool ones on there, like getting through levels without scoring a single point (so even coins and mushrooms are the enemy), or Goomba combo-stomping.
Should be good fun if you want more out of the already brilliant platformer. It's a shame, in the age of Achievements and Trophies, that Nintendo is doing it this way instead of just building these challenges into the game itself.