I _don't_ think the right answer is anything goes (even if you attempt to let the community police that with voting). I've seen many small forums self-destruct due to the dedicated action of a few committed individuals to ruining it -- either explicitly (due to a grudge) or implicitly (a commitment to getting their way at any cost). Misinformation is terribly difficult to combat, and can lead to real harm to real people.
I _don't_ think the right answer is heavy moderation. I even feel like hacker news goes overboard sometimes (though I do enjoy the irony of comments here railing against calls for moderation on FB). A ministry of truth is never a good idea because it creates a clear path to corrupting the truth by controlling an entity.
I do think it is reasonable, however, to consider certain things to be settled facts. Humans breathe air, the earth is round, the holocaust happened, slavery is bad, racism is bad. Of course I have no answer about where the line is drawn between what I just said and a ministry of truth.
In my experience, the most successful communities are intelligently moderated, however, including IRC. Though the moderation was more about banning people and keeping channels on-topic than suppressing certain ideas.
The best answer I have so far is social consequence -- disagree vocally & publicly, block early, block often. Of course there are a whole slew of downsides here too, but I haven't seen anything better.