When Reddit puts something on the hot tab, there's one hour or two for anybody to say what they want to say, no time for anybody to reply, and it's gone. HN is slower, but not very different. It's a completely different kind of site.
Used to be in alt.callahans and I can't imagine how many threads I just collapsed and moved on because I knew that conversation wasn't a topic of mine. And still managed to ping in at least a few dozen messages of varying size/quality a week.
There are huge benefits to this like minorities gaining scale to be heard. But I feel a lot of benefits are of the type of taking something very good and scaling it up to a global level. What is less seen is the destruction of a lot of other good solutions elsewhere which might have had a chance to develop into something even better with a little more time. We are loosing diversity in products and ideas.
Finding ways to restore smaller scale in some areas may be beneficial but it is not clear how to make it work. Arbitrage across boundaries levels differences. The benefits of tearing down the boundaries accrue with the rule breakers who gain scale. Google+ was an attempt to stem the tide. It may have been flawed but it also tried something fundamentally hard.
Back in the late 90s early 00s I was a member of a forum for a chain of video game stores in the UK, it was an amazing community. The store went bust in ‘05 and one guy created his own forum software where most of the displaced members flocked to.
That forum is still going, although there’s probably only 10 or so regulars left - but we still talk on it every day. I’ve even met a few of them in the real world.
EDIT: I think you've hit the nail on the head, the thing that was so great about forums is, while the original point of the forum was to bring together people with a like minded interest (video game(s)/whatever) - the forum would also have separate boards to discuss cross over topics or something completely unrelated. This meant you were talking to people who you might recognise from other areas of the forum and share other common interests, so could build that sense of community.
Whereas reddit just feels more siloed, I wouldn't recongise anyone who visits subreddit X who also happens to be on Y etc