Earlier quoted context omitted.
Most subreddits I visit are low volume and haven't noticed much difference in behaviour other than acceptability of short funny replies. What do you mean about the format, isn't it the same as HN (OP link/text and comments)? I do recall a comment/thread saying how HN comments/behaviour was starting to resemble Reddit, so there may be something to that (though I'd chalk it up to becoming more popular/mainstream) .
HN only works because of supplemental rules and heavy moderation, which both look very different on Reddit. Even if a subreddit reaches a good set of rules, enforcement is usually poor, due both to mods being volunteers and a generally negative view towards moderation (can you imagine "Mods are gay" flying on HN?) Any moments of "lightning-in-a-bottle" can certainly result in memorable periods where a subreddit truly…
I would mention /r/credibledefense as a counter example. It was a niche subreddit before the Russian invasion of Ukraine, in the aftermath got popular, but still managed to keep its quality.
But I agree with your general point, it's not a huge problem for me, though. Some subreddits get popular and thus bad, some new subreddits get created. Reddit's relative anarchy is its saving grace, communities can split/fork, but still stay within reddit platform.