What "Complexity" Actually Measures (It's Not Page Count)
Complexity = how many decisions the rules require per minute of play. Not how many rules exist.
The community distinguishes three axes of complexity: character-building depth (how many choices at creation and level-up), tactical decision density (how many valid options per turn in combat), and procedural overhead (how many rules you need to reference during a session). A game can be high in one and low in others — Cairn has no character-building choices but high tactical decision density because positioning and improvisation matter so much.
The trap many tables fall into: equating rulebook thickness with game quality. A 400-page rulebook that you reference twice per session is less "complex" in practice than a 50-page rules-light game that demands constant DM judgment calls. The real question isn't "how many rules?" but "who carries the cognitive load — the book, the DM, or the players?"
Lightweight Systems: When Less Is More
Light games work best for one-shots, new players, and groups that trust the DM to make fair calls.
Rules-light systems (Cairn, Mork Borg, Knave, Lasers & Feelings) share a common philosophy: the rules exist to resolve uncertainty, not to simulate physics. Character creation takes minutes. Combat is fast and deadly. The DM's judgment is the primary mechanic. These systems shine in one-shots and short campaigns where momentum matters more than mechanical depth.
The community's key insight: lightweight systems demand more from the DM, not less. Without rules to fall back on, the DM must make consistent, fair rulings on the fly. This is a skill that takes practice. New DMs sometimes struggle because they don't yet have the judgment to make good calls; experienced DMs thrive because they can finally run at the speed of thought.
Crunchy Systems: When Rules Are Features
Heavy systems reward system mastery. They work for long campaigns with committed, build-loving players.
Pathfinder 2e, GURPS, and Shadowrun represent the crunchy end of the spectrum. These systems have rules for situations that lighter games leave to DM fiat — and those rules create gameplay that couldn't exist otherwise. PF2e's three-action economy creates a different tactical landscape than 5e's move-action-bonus. GURPS's one-second combat rounds make positioning and facing matter in ways that six-second rounds can't.
The community is clear about when crunch works: long campaigns with consistent attendance where players enjoy engaging with the rules between sessions. If your players read rulebooks for fun, crunchy systems reward their investment with depth that lightweight systems can't match. If your players show up, roll dice, and go home — stick to lighter systems.