In “A Quick Primer for Old School Games”, the author Matthew J. Finch lists four “Zen Moments”, principles almost diametrically opposed to modern role-playing game design but which make sense in context:
Rulings, not Rules. Old School games have few rules, mainly for important situations and concepts. In all other cases, the Referee should make a decision on the fly, based on logic and common sense. Referees can shape the world according to their own preferences, and don’t check “official rules” for every situation.
Player Skill, not Character Abilities. Old School games by and large eschew skill systems. When a character needs to find a hidden object, the player describes where he’s looking instead of rolling against a “Spot Hidden” skill. Old School players claim that this style of play leads to a more immersive experience.
Heroic, not Superhero. Players characters may start off with abilities “ordinary people” in the world don’t, but they are far from invincible or unstoppable. They can fail, they can get hurt, and they can die. Building a character up from slightly better than average to a mover and shaker – who can still die under the right circumstances – challenges the player more than the typical modern character, who even at the start takes a lot of effort to kill.
Forget “Game Balance”. Referees need not balance player characters against their opposition, or against each other. The game world need not be “fair”; it’s a story told among players and referees, not a tournament or wargame. Likewise, a Referee should not abuse his power by overriding a player’s choice of actions. As long as everyone is having fun, who cares about power imbalances or encounters that are “too hard”?