Don't be a Jerk. Griefing is unacceptable and griefers will be dealt with on a case by case basis.
Communicate: Most of the following rules can be read with "Unless you have the expressed consent from all players involved" tacked on. Accidents happen but if you don't tell anyone it looks very much like griefing in the logs. The logs show everything... Everything.
Building in the Water: See above threads for claiming land. If you want to build over water, that can be arranged.
Signs: Use them. Mark everything you build. The more info explorers of our world have, the better.
Keep it friendly: Structures shouldn't be more then PG-13 rating.
PvP: PvP is only allowed when agreed upon by all players involved including the Land Owner. Do not attack someone unless that player has agreed to combat.
The Nether: Build in the Nether at your own risk. This does not mean PvP. This means it is very possible that a Ghast shooting at an explorer passing by can mess up your creations. Pro Tip: Ghasts can not blow up Stone/Cobble