1. Be respectful. A.k.a don't be a jerk, no sexism, racism or other -isms. Do not bully and so on.
2. No griefing or stealing. I mean it shouldn't have to be stated. Mobgriefing is no however and if the accident were to occur on your fellow community members building or land. Please clean it up to the best of your capabilities.
3. No cheating, hacking or abusement of bugs (such as duplication). This is a vanilla SMP server for a reason, if you wish to take shortcuts, maybe this server isn't for you. Any smart use of intended mechanics is encouraged however.
4. Do not build too big too close to spawn, and no main bases directly adjacent or at spawn. This is to allow community buildings and give way for any community projects. Spawn is supposed to be for everyone not just you. As a rule of thumb, the farther away from spawn you are the larger land you can grab. Any land you have not altered is not considered yours, unless agreed by the community.
5. Do not abuse the server, do not cause too much lag. It will effect everyone.