The sheer amount of propaganda on Reddit is insane. You could take a random selection of titles off /r/popular and repost it to the /r/circlejerk of 5 years ago and it would've been a top post. I think that's part of the reason for its declining popularity: it's hard to make more absurd posts than what actually becomes popular nowadays.
Every community develops its own biases, HN included, but Reddit is in late-stage groupthink. It wouldn't be a bad thing if what got traction wasn't doom and gloom. So many posts boil down to "the world is a mess"/"I am a mess"/"other political side is so stupid how can other people think different?"/"hate this person for expressing X opinion or doing Y (at least for like 3 days)".
It's not just an online thing only, I've seen many people who have internalized some of these beliefs since they see them expressed so much online. Being ruled by what the internet thinks is acceptable is a horrible way to live your life, but if you're an awkward teen you might very well end up like that.
Maybe I'm just nostalgic but the early internet had this care free spirit where every interaction, good or bad, seemed somewhat fun at least since it was crazy you were able to at all, and there were no stakes. People would flame you for your political opinions but you'd just shout at each other until a moderator told you to shut up, not have an entire community go against you since you don't agree with them. It was fun!. This "ganging-up" OTOH induces the brain to start modifying its beliefs to fit the group. A very normal process, but harmful. And younger people, the audience of Reddit, are exactly the most vulnerable to this online peer pressure.