Personally, I think that this kind of system would greatly detract from the experience of a game. Example: You're playing Zelda U, a game that will be hugely atmospheric, and you feel immersed in the game, so much that you forget that it's a game. Then - 'Achievement unlocked: Kill 100 enemies!' It would remind you that you're just playing a game, and not having an experience. You aren't Link anymore, you're just controlling him. Besides, collecting all heart-pieces is an achievement, you don't need a message on screen to tell you that you've done it. Nintendo have always been about personal achievements, so why change that? Miiverse is so much better.