Critically, Slashdot also had a meta-moderation system, where users were asked to judge moderation activity to confirm whether it was sensible, fair, and so on. I'd like to believe that system played a vital role in stopping abuse of the moderation system. It was way ahead of its time.

I've been advocating for a while now that HN could use meta-moderation at least on flagging activity, so it can stop giving flagging powers to users who are using it for reasons other than flagging rulebreaking.

Reddit awards one karma for a comment if it doesn't get downvoted. I noticed the other day that I got a pretty random and only tangentially relevant comment on a one month old post I made. I checked out the user, and they were only commenting on old posts to slowly accumulate karma. Only the poster will be notified about such a comment, and as long as it is made to be made of platitudes, most people will not bother downvoting.