Shoot someone in the head in Morphies Law and you won't kill them. Instead their head will shrink and yours will swell. Shoot him in the legs and, again, their limbs will shrivel while yours grow. It's a simple mechanic but I love the implications.
I like games that handicap players as a byproduct of their success. For instance, in drinking games it should always be the winning player who drinks so as to impair their ability to keep winning; drinking games that punish losers just increase an already present skill gap. Morphies Law captures that dynamic perfectly. If you're good at shooters and able to pull off headshots with a twitch of the wrist then you'll find your head grow, and grow, and grow with each successful kill, making it a bigger target for your enemies.
I like, too, that if you are good at the game you will literally become a giant among men. You can tell a player's ability and even something of their playstyle from their body shape. If they're eight-foot-tall with a massive torso you know they tend to aim for the chest.
Take a look at it in action:
Morphies Law will be headed to Steam Greenlight but there isn't a release date yet.