Unwritten rules that I follow:

1. Always play with more than 1 monitor. Alt+tab is a good way to miss things.
2. Unless stated in party chat, tank is leader, a DPS who decides he can't wait and pulls is most likely a dead DPS.
3. Be courteous when giving advice. "Git gud" is not advice.
4. Having a DPS meter is ok. Using it to break rule #3 is not.
5. Not liking a type of content doesn't make it "useless". People might be surprised that other people might enjoy that type of content.
6. Don't call any role or job "easy". What's easy for one person is not necessarily easy for another, but that doesn't mean the other person shouldn't try. If they really try, they'll eventually get it, it will just take some time.