Game design, it's very complicated.

Reading Raph Koster's opinion on the difference between rules and mechanics, and from what I gather he says that rules are parts that make up a mechanic. He uses the action of jumping in a plat-forming game as an example: rules are things like gravity, inertia, discrete values that can be removed or added, mechanics are the "black box" that encompasses these rules. Mechanics would take in player interaction from dynamics, in the Mechanics->Dynamics->Aesthetics framework of games, and then using the discrete rules, is able to vend out a value, like say the player will jump 24 pixels up after taking in gravity and previous velocity.

More deadly things in one screen than I wanna be the Guy. Rules wise the enemies have a radius of 1.0f or a bounding box of size 24x24, mechanics wise getting close to them means death!

A game designer needs to have a grasp of things on the atomic level of the game, made up of floating point numbers, boolean, and integers. Tweaking these things tweaks the rules of the game, maybe make a monster easier to dodge by shrinking the collision box, or make the player walk slower by turning down that floating point number which controls speed. The game designer can spent hours tweaking these values (the above game is a tuning game, where you can tweak some numbers to clone jumping aesthetics in a game like Mario) to get the feel they want for the game.

We should probably have a playable game first though...