Thoughtful extra points added by Stuart Allinson

4 Critique the idea not the person. 

Just like a board meeting, the members of any forum need to set that standard and call it out - but not by tribal exclusion, but by encouragement and inclusion. There are many ways to do this - too often the ad hominem argument hides an important idea that is poorly expressed, or a flawed idea whose logical implications have not been fully worked through.

..and maybe a rule 5 that requires a little more skill:

5 Humour helps!

Again, not at the expense of a person - humour can defuse a heated debate, but most importantly, it creates the space in which people can engage to explore "dangerous" ideas. But it must be done with respect