Bone attempts Nuzlocke

Any Pokémon that faints is considered dead, and must be released.

The player may only catch the first Pokémon encountered in each area, and none else. If the first Pokémon encountered faints or flees, there are no second chances. If the first encounter in the area is a Double Battle in dark grass, the player may choose which of the two Pokémon they would like to catch.

While not exactly a definite rule, the general consensus is that players must also nickname all of their Pokémon, for the sake of forming stronger emotional bonds.

Also not a definite rule, but the general consensus is that a black out/white out is considered to be "game over", even if there are Pokémon left in the PC.

Strongly implied, though not explicitly mentioned in the comic, is the stipulation that the player can use only Pokémon they have captured themselves, meaning traded Pokémon, mystery gifts, etc., are prohibited. Trading and retrading the same Pokémon (for the purpose of evolving a Graveler, for example) is something of a gray area, and may fall under optional rules. As of White: Hard-Mode Episode 3, it is implied that the player can accept Pokémon that are received freely from NPCs.

Also strongly implied is a prohibition against voluntarily resetting and reloading the game when things go wrong. Being able to do so would render all of the other rules pointless.