Mechanics vs dynamics

A mechanic is a rule you explicitly design: "the player can place a bomb that explodes after 3 seconds." A dynamic is the behavior that emerges when players use mechanics together: "players kick bombs toward enemies and chain explosions." You design mechanics directly; you can only encourage dynamics.

The best game design gets a lot of dynamics out of a few simple mechanics. Chess has a handful of rules and endless emergent strategy. When designing, ask what interesting situations your mechanics create together — that is where depth comes from.