I've spent a lot of time on old-style (phpBB, SFML) forums of all sorts, and I feel that Reddit and HN are an improvement on them - but there are two factors to disentangle here: discussion shape, and discussion cadence.

In terms of shape, I very strongly believe that tree model - like on Reddit and HN - is superior to the linear model.

Every discussion naturally keeps trying to sprout new branches. People go off tangents, or mention multiple distinct points that others then try to elaborate on[0]. Tree-shaped discussions support this naturally - tangents end up branched off as subtrees[1]. In contrast, the linear model only supports one main thread. Any tangent is either short-lived or takes over the main thread. Attempts at carrying multiple tangents can easily destroy the whole thread[2]. And you have to read through all of them, in sequence, mentally keeping track of their evolution, even if you only care about the (current) main topic.

So, UX-wise, in my opinion, the ultimate discussion platform has to be non-linear[3].

In terms of cadence, I'd also like to see a platform that encourages longer discussions, conversations happening over weeks or months. Forums ain't it - of the ones I've been on, all considered replying to a week-old conversation as "necroposting"[4]. Yes, individual threads can last many months - but these are not really single discussions any more. The limitations of the linear form mean that people can't jump in and fully participate - they'd have too much material to work through. The tip of the thread becomes its own living thing, bearing little relation to its topic, or 90% of its content.

The kind of platform I'd like to see would allow the discussions to grow over time while remaining readable in entirety, so that a topical thread could accumulate knowledge. That necessitates a non-linear form, and possibly some meta-level UX for abridging long conversations. And, of course, a notification system (which HN, notably, doesn't have), so you could track the old threads as they're displaced from the front page by new ones.

Culture-wise, this would necessitate the reversal of traditional approach to necroposting. Where forums don't like reviving threads, and HN won't even let you (it won't pop back on the front page just because you commented on it), this new kind of platform would have to encourage people to find and continue previous discussions instead of starting new ones.

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[0] - Case in point: this comment. My response covers two distinct topics: tree vs. linear model, and cadence.

[1] - Going back to [0], someone can reply to this comment and talk about just one of the two topics, someone else can reply to cover the other - and now you have two separate, focused subthreads, that can evolve independently and don't interfere with each other.

[2] - You've likely seen plenty of discussions where, at some point, replies start to consist mostly of quotes from previous replies, covering several different topics. This quickly gets impossible to trace and people just either drop most of the tangents, or stop replying altogether. It can sometimes stabilize to the main branch covering 2-3 topics simultaneously - through quotes, or alternating replies.

[3] - That doesn't necessarily mean trees. A question I've learned to always ask myself when modeling things as trees is, what if we used a directed graph instead? I haven't seen this tried, but I think DAG-based discussion would be better. A problem with tree-shaped threads is that you can't merge subthreads back when their discussion converges. You can see this frequently on HN: unrelated top-level comments end up each having the same discussion 2-3 levels down. Even this very thread. DAG-based UX could help reduce that duplication, and as a bonus, it could offer better experience for cross-references between comments.

[4] - See https://news.ycombinator.com/item?id=27274439.