Are you perhaps arguing that as long as people don't have a unified formal definition of niceness, nice behavior is not possible? That would seem unlikely.
Even if everyone has a different threshold... well, everyone has their own upvote and downvote buttons, right? So the worst case is that some comments would get upvoted by some users for being nice enough and downvoted by other users for not being nice enough. Doesn't seem that horrible.
And over time, people will adjust to the average. And those who will still find this community unbearably rude or unbearably polite will leave.
In real life, this problem is usually solved by creating subcultures; different groups having different norms. Being too rude will get you ejected from the group. Being too polite may make you leave the group voluntarily. Groups that eject too many people end up have few members. Groups that retain too many rude people end up having mostly rude members.
It would be a nice experiment to have a website that would support this "organic" grouping of people; where LW wouldn't be one group, but rather an ecosystem of groups. But I'm afraid we are unlikely to ever see this happen. So we are stuck with having LW as one group.
In real life, sometimes the ejecting of rude members from the group is done by a local boss (a formal owner of the place, or a high-status member of the group), but sometimes the group splits "organically" -- some people stop talking to some other people, and after some time we see that what was originally one group now became two groups. It could be interesting to try modelling this by a web platform. (Mere blocking is not enough, because in the group other people see when X is ignoring Y. Also, avoiding someone in real life is not a binary decision.) But I am not expecting to see this in near future.
So we are stuck with having LW as one group.
I don't think that LW is one group in any meaningful sense. There's this website. There's Slack. There's IRC. There's the facebook group. There are local meetups with often have their own mailing list.
There are also various diaspora groups that don't exist under the LW brand.
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