Just because most people are decent doesn't mean most comments are going to be. A decent person might right out a thoughtful post at one site with the same freq

Just because most people are decent doesn't mean most comments are going to be. A decent person might right out a thoughtful post at one site with the same frequency that it takes a misanthrope to write 100 comments across multiple sites. And once something starts to become a shouting match, you'll get a certain number of people who go along with things even if they wouldn't act that way normally (since they think that's what most people are doing, even if it's just what a loud minority is doing). But with good moderation you can have good conversations. For instance, I've found subreddits like /r/AskHistorians to be pretty good (for the most part).