Don't grief. Don't destroy, deface, or steal from something someone else built. This is a small world; everyone knows who built what.
Don't cheat. No hacked clients, no x-ray, no duping, no glitches-that-are-clearly-glitches. Grim watches, CoreProtect remembers, and neither forgets.
PvP stays off unless both sides agree. Fights are fun when they're wanted. Killing someone while they're mid-build is a quick way to lose your whitelist.
Be decent in chat. No harassment, no spam, no personal attacks. If you wouldn't say it face to face, don't type it.
Respect spawn. Keep builds tidy, leave the spawn area open, and fill the holes you dig. It's everyone's first impression.
Don't be a resource hog. No AFK contraptions or redstone farms that grind the server for everyone else. It's a small box with a job to do.