Technically we could be all writing in a giant Notepad file, and adhere to any discussion format.
(My experience is more than 10 years old, though)
In email, top-posting began when Microsoft’s first email client placed cursor and signature at the top of the quoted email instead, and didn’t provide commands to reflow partial quotes, or any of the features mentioned above. It also had no threaded view, which is what makes it practical to only partially quote instead of fully.
Culture is important, but technology can influence it heavily.
Reddit, and the even worse lesser forums, loses pretty much all of that. Browsing reddit, for me, feels more like watching a mix of a river with flotsam drifting by, a busy traffic street, uncoordinated fireworks, and a tornado ripping through a midwestern city. There are no tools to track what you have and haven't read already, or what new comments have appeared. You cannot sort and filter the posts properly, the best you get is a "do you feel lucky?" search, which often shows that "no, you weren't lucky today". On low-traffic subreddits, it IS possible to track new stuff, but you have to do so manually. I offer no solutions, I don't know how to effectively do highquality discussions for 6 or 7 billion people.
That's both a matter of design and scale. Forums larger than ~10^2 -- 10^3 participants aren't really discussions so much as a compilation of hit-and-run pieces. For very large subs (10^5 -- 10^6), discussion is effectively over within a day, if not hours.
A strong contrast are the now-defunct Google+ and (at certain points in its evolution) Ello, and the not-quite-dead-yet Diaspora*, all of which had or have a "notifications" pane in which recent discussions are presented in full, and to which all or most prior participants (so long as they've not muted the discussion) see not only direct responses but new comments. I've seen specific conversations continued for days, weeks, months, and even years, productively, and it's a really good way to noodle at an idea (particularly with a good post moderator) over time.
There are other factors about that which also contribute. All three of these platforms are post-and-comment style (HN can be somewhat like this), where the post author is the moderator of that post (HN is NOT like this). I consider the post author a "host", and think of this as a "salon-style" network. Barring blocking actions (which can have a pretty profound impact, TBH), everyone is mostly seeing the same discussion.[1] All three platforms also showed comments to posts in strict, flat, chronological order. As a long-time user of threaded interfaces (tin, mutt, /., Reddit, HN, etc.) this grated strongly on me for quite a while, but I eventually came to see the model as useful and with merits. It's an option and a tool in the box, though I'll still say it's not appropriate in all cases.
I'll also note that G+, Ello, and Diaspora* all have relatively small limits on discussion, with a post permitting up to 500 comments, so while the platform sizes could be large (G+ claimed over 4 billion "accounts", though highly actives were ~10 million or so), individual discussions tended to be fairly small. I'll also note that the salon-style engagement was fairly rare, many people (and institutional/organisational profiles) treated the platforms as post-only or broadcast, and my experience may well be niche. That said, there were some awesome discussions on all three of these which simply don't seem to gell elsewhere: Reddit, HN, and the Fediverse come particularly to mind.
Specific to HN: it's ... close. Discussions are threaded and moderated (both by mods and members, though by different mechanisms). There's no notification of follow-ups and it's hard to tell when a conversation's really died, or if it's still active. Participation quality is ... mixed, but there are some occasional gems. I do regret that there aren't better tools to surface really-high-quality comments or threads, though there've been some manual processes in the past ("best of" lists). That effort's largely been dropped, for understandable reasons of time and scale.
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Notes:
1. Some day I really should write up my thoughts on blocking. Effectively, it seems to create a geography or topography of the social network, which combined with follows and inter-participant links gives paths of greater and lesser propagation. Blocking by highly-linked nodes in particular can be devastating to reach, particularly in a simple chronological timeline. I'm a fan of the mental-hygiene aspects of blocks,[2] but have to admit that being on the receiving end can occasionally hurt. But beyond that, the effects on how following and blocking shape the overall network and activity dynamics is simply fascinating to me.
I vehemently disagree that Reddit is in any way even approaching equivalent, much less better, than Usenet was.
There's something about the text area, vs an email-like posting box, which encourages low-thought short replies.
I think worshipping Usenet is just a simple case of rose-tinted glasses. Like people telling how awesome old OSs and applications were in the 90s ;)
I'd had my first tastes in the late 1980s, where some uniformity of tools, platforms, and cultures (tin, Unix, and largely uni-based participants) tended to cohere. I've cited on HN Brian Reid's Usenet activity surveys (from John Quarterman's The Matrix,[1] an early exploration of what we'd now consider social networks) several times on HN[2]. As of 1988, there were 381 newsgrous, 1,933 articles/day, 4.4 MiB/day of traffic, 7,800 hosts, and 141,000 readers. That's ... tiny by contemporary standards.
Cultural norms broke down rapidly as Usenet spread first to corporate networks (Lotus Notes email formats remain a massive annoyance etched in my brain), and then the general public. By the mid-1990s, Usenet though far more active and reaching far more people was a pale shadow of its former self in terms of culture and relevance.
The fundamental technical presentation along with the original posting culture was fairly effective. That unfortunately didn't scale. One of my concerns as I look at decentralised networks with multiple clients and server implementations (e.g., the Fediverse) is that a diversity of tools will inevitably result in a broken set of standards and practices. In the case of the Fediverse, the baseline is fairly low, though there are implementations and/or instances which offer some fairly narrowly-supported capabilities, notably raw HTML or Markdown formatting, and equations support (as with ColinWright's Mathstodon).
I'm not arguing against your experience, FWIW. I am arguing that your experience is probably highly time-dependent on when you participated in Usenet, and the era from 1979--1992 is markedly different from that of 1993 onward, though cracks were already starting to show.
I've also noted that HN has survived longer than Usenet's golden age, and with remarkably stable quality. It's not all it could be, but it's not the worst of what's online either, and by a long shot. Despite some frustrations, I still find it useful.
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Notes:
1. <https://archive.org/details/matrixcomputerne0000quar/page/24...>
2. Search shows most, excepting where I've brainfarted "Eric" for "Brian": <https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...>