The differences aren't chiefly technical. They are cultural, as you alluded to. I started out using BBSes then usenet and email lists then forums and now reddit, for an overlapping set of hobbies. I can tell you with certainty that the subreddits about those hobbies are a pale shadow of what existed on ANY of the prior discussion platforms.

The volume of conversation might be higher, but the depth and sophistication is lower. The repetition of clueless questions. The endless posting of the same joke responses rather than actually answering questions. And so on.

All of that stuff existed on forums and usenet and other places too. I'm not saying it didn't. It's just that the proportions have shifted. And I think like you said the friction is part of it.

It's not just the interface. It's that effectively "everyone" has an account on Reddit. So if they stumble into random niche subreddit because the algorithm suggested it or someone linked to it or it popped up in a search result, in two clicks they can be posting their own new posts or replies in that very niche community. With standalone forums, it was both less likely that you'd just stumble across them if you weren't specifically interested in the topic, and the bar for starting to participate was much higher.

Even if there were no real restrictions on joining or posting, just creating a new account is a lot more work than participating in a subreddit when you already have a reddit account. You could argue that the same dynamic existed in usenet, but the overall bar for participating in usenet was so much higher, and the global userbase so much smaller than what reddit has. And still, we did in my experience see a lot more of the kind of garbage participation that comes from people who aren't really interested in or knowledgeable the topic being able to participate with zero marginal effort.

An extremely low barrier to participation creates a radically different culture than a situation where you actually have to want to be there before you contribute.

It's not just about how many people are on the internet now. There are still a handful of niche forums I participate in, and maybe they aren't as good as they used to be, but they're still way better than most subreddits.

This. It’s the core difference in forums causing the discourse to be higher quality. They were all communities after all. Some, I admired and learned from as a mere visitor as I knew my contribution wasn’t additive. So I just never joined, Reddit invites all to engage in the “discussion” which devolves quickly to essentially potty humor.

Some forums, I joined and contributed and became a member of that community. There were friendships and personalities behind the usernames. They sprung into IRL friendships in a few cases. On Reddit, I hardly ever see the same username twice. It may as well be fully AI generated, I wouldn’t know the difference.