Backwards: One or more aspects of the game works the opposite as would normally be expected: maybe the controller is reversed (left is right, right is left), maybe you win lives every time you die, maybe it scrolls to the left and your character moonwalks.
No cheating (please): The game system relies on willing player participation to enforce the rules of the game world (e.g., hold your breath, close your eyes, etc.).
Bigger Picture: The game touches on a political, environmental or social issue.
May the (Web) Force Be with You: The game runs in a browser with no plugin.