The lack of subreddits is a big difference because it inhibits users coagulating into subgroups. Reddit also allows people to be banned from subreddits by subreddit owners/mods which leads to groupthink more.

HN is one large subreddit with it's own groupthink.

The content is also different. Reddit uses AI agents to populate feeds to generate user engagement.

The main difference is no personalized algorithm ranking content to increase engagement.

The algorithm has consumed reels/shorts/tiktok/insta. Being extremely inflammatory is rewarded there.

Algorithms don't define social medias, it's just the latest iteration from the main ones.

Is BeReal not a social media because there's no algorithmic feed?