Tildes is a good example of a healthy community that allows for differences while encouraging good faith discussion. They police for tone instead of wrongness and it's been working out over there. People are generally happy with the discourse.

A lot of it is in site design, too. There aren't downvotes, because they're not needed. There's a lot of proactive moderation coming from the community by using comment labels. Labels help push comments up or down, and some require you to type a reason why, which encourages thoughtfulness instead of knee-jerk hivemind reaction and pile on. The only publicly visible label is the "good" one, so it keeps things positive. The "bad" label alerts mods and has a cooldown time limit, so it's less likely to be abused. I believe once it's used on a comment, the person can no longer reply to it, which helps avoid negative back and forths.