Half the requests you will get in the first month are game rules. They are one command each, they take effect instantly, and a couple of them change the character of a world more than people expect.
Difficulty is not a game rule
It lives in server.properties as difficulty=easy|normal|hard, or you can set it live with /difficulty hard. On peaceful no hostile mobs spawn at all and hunger does not drain — which quietly removes most of the reason to build anything defensive.
The rules people actually ask for
| Command | Effect |
|---|---|
/gamerule keepInventory true | You keep your items on death. The single most requested change, and the one that most changes how careful people are. |
/gamerule mobGriefing false | Creepers stop cratering builds; endermen stop stealing blocks. Also stops villagers farming. |
/gamerule doDaylightCycle false | Freezes time. Useful for a build server, strange for survival. |
/gamerule doFireTick false | Fire stops spreading. Worth it on any server with wooden builds and a griefer. |
/gamerule randomTickSpeed 3 | The default. Raising it speeds up crop growth and hurts performance quickly. |
/gamerule announceAdvancements false | Quietens chat on a busy server. |
keepInventory is hard to walk backTurning it on is easy and popular. Turning it off later, once people have got used to deep mining without consequence, is the change that starts arguments. Decide it early and say it out loud rather than switching it under pressure.
Game rules typed in the console apply immediately and are recorded in the log. It also means you do not have to op yourself just to change one — a small habit that keeps your operator list short.
/gamerule keepInventory with no value prints the current setting. Reading it back is faster than testing it by dying.