My entry's "only one" is the player's only one ability, which is to jump (more than once, though). And, also, the game's playable with only one button.
I intended to have more stages and a recurring boss that you would have to fight in different ways each time (and an one-legged character for extra "one-ness"), but
the deadline lazyness struck me hard. Still, I'm glad I managed to submit something in the only one (HA!) Game Jam that I participated.