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If you run a forum and actually use it, then if someone attacks you, you're presumed to have seen it. Which means if you don't reply, it reads as a tacit admission that the attack is valid. Which creates really bad incentives for people.
This is an edge case that few other people will encounter, but when you run a forum and write essays, it's like having comments on your site, even though you didn't want to.
I see.. It would be interesting if Twitter 2.0 programmatically implemented a sense of community/mutual respect - slashing points of users if the community or the product deems you're being spiteful. Like what would happen IRL if you said something nasty to someone in a community
I'm sure there'd be some unintended higher order effects with that, harmful or pleasant. It would make for a cool experiment regardless.
Also now I realise a lot of your criticism of Twitter is to spark the founder that will build the product you want! 
Do you still think of HN as your forum?
No, definitely not.
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never thought of that last point... What are the issues you'd face exactly? That you'd attract unwanted attention and not let organic discussion happen?