There's only one way I've seen in which one can reliably make and maintain a polite community: build it around a niche topic and moderate away things that stray too far away from it. The moment you let your community be about anything and everything is the moment the "unwashed masses" come and the whole thing starts catering to the lowest common denominator, which is a pretty shitty level.

It's not a complete recipe, but I feel it's a necessary condition. This is a big part of why HN is a polite place, or why most niche subreddits are polite places.

Your forum could still be "about anything", yet not cater to lowest common denominator.

It's only when you deliberately want to attract (and retain) as many eyeballs as possible, then the quality necessarily has to go out the window.