The theme of the game is “be consistent”. It's a minimalist-styled 2D platformer. The core mechanic is that whatever you do the first time, the game makes it so that that was the right action. Examples of how this could work:
At the start, you're standing at the center of a 2×2 checkerboard of background colors (plus appropriate greebles, not perfect squares). Say the top left and bottom right is darkish and the other quadrants are lightish. If you move left, then the darkish stuff is sky, the lightish stuff is ground, and the level extends to the left. If you move right, the darkish stuff is ground, and the level extends to the right.
The first time you need to jump, if you press W or up then that's the jump key, or if you press the space bar then that's the jump key. The other key does something else. (This might interact poorly with an initial “push all the keys to see what they do”, though.)
You meet a floaty pointy thing. If you walk into it, it turns out to be a pickup. If you shoot it or jump on it, it turns out to be an enemy.
If you jump in the little pool of water, the game has underwater sections or secrets. If you jump over the little pool, water is deadly.