GNS and Gamemastering
The concept of GNS (Gamist / Narrativist / Simulationist) as a tool for guiding game design is well-documented on the internet on such places like The Forge in discussions and articles. Gamist play normally involves a clearly defined set of "victory" conditions, and rewards and consequences for actions. Narrativist play normally focuses on the story above all else (as determined by all the players). Simulationist play deals with exploring the plausibility / realism of the world or setting of the game.

Just yesterday, I was struck by the fact that the initial GNS balance as set down by the rules set actually changes when in the hands of a Gamemaster (GM). A GM's natural biases will twist the game along the three axes of the GNS paradigm to fit his or her taste. GMs with a strong narrative bias will pull the game towards that axis... same with the other axes. The system and the setting will still define the initial starting point... and in some ways reinforce that style of play. But the GM (and the players) will eventually make the game their own.