Reminded by this Eurogamer article on achievements in general and the game Achievement Unlocked specifically, I came up with one of my many throwaway design ideas (which I hope to turn into a blog series).
I hate achievements. There, I said it. If not because of the fact that they are required to unlock Team Fortress 2's new weapons (which are invariably not that useful) , or they are mostly the province of those showing off their gamerscore, or that they promote unfun behavior as much as fun behavior, then that I simply don't like the idea of them at a basic level.
Why do we need little blurbs popping up on screen to tell us we're having fun experimenting with gameplay? Well, as much as I can't stand them I understand that people like them. Inspired by the popularity (and the fun I had playing it) of Achievement Unlocked and people's endless ability to grind for achievements I thought why not harness their power for good.
The result: create a game which is entirely about earning achievements which themselves are entirely secret. The whole point of the game would be to try as many novel and interesting combinations of actions as possible.
In this case a physics system or a sandbox world would really help things out, as well as the possibility of user-submitted achievements. Taken to a ludicrous level, every action the player had not tried before could result in an increase in score/receiving an achievement.