> Now, you could argue that lots of topics on a forum is a "kind" of threading...
Yes, not only could I, but I have in all of my posts here so far, very explicitly. ;P A "classic web forum" has four levels: categories, forums, threads, and posts. It is critical to look at a web forum as having all of these levels if you want to compare it to a site like HN, as otherwise it is impossible to discuss how end users are able to navigate the site.
As an example, let's say that we were going to talk about your article on a normal web forum. First of all, that forum may already be divided into "on-topic" and "meta" forum categories, and there may then be multiple forums that are interested in your talk: the conversations don't conflict.
However, even within the same forum on the site (which will often then have the same general participant pool), your article will spawn multiple unrelated threads, each of which will consist of linear posts. Each of these threads will be discussing different areas of the topic, and you can ignore the threads you aren't interested in.
Hacker News can't do that: your article can only create a single top-level link, and all conversation has to exist underneath that. You get the same levels near the top (although managed in a distributed collaborative fashion by the users, as opposed to directed by the site operators), but additionally as you drill down it might get very deep.
... and, once again, I will agree with you that HN has a problem managing the discussion near the leaves of the tree. (FWIW, I actually was not allowed to reply to your post, but decided "screw that" and did it anyway: it could be nice to see something linear once you get this deep instead of just turning off "reply" altogether.)
> If you can point me to any web discussion communities that are 10+ years old which use threading, I'd love to see them.
In addition to classic web forums, which I maintain are the typical 4-5 level hierarchical tree, you will find websites like Slashdot, which is definitely a 10+ year old web discussion community, and is insanely threaded with tons of awesome mechanisms to handle that complexity (such as the feature of "bubbling up" good comments I mentioned elsewhere in this thread).
> but I'd never, ever see it because it is forever married to its place in the tree and the iron-clad parent child reply hierarchy
The solution reddit has for this--both in the commonly-used Reddit Enhancement Suite and as a feature you can buy as part of reddit Gold--is to mark new sections of the tree in an obvious way (such as with a blue box). I believe DISQUS does this as well now, and I have a similar feature in the HN script I sometimes run. In a system like USENET, you would have the option of resorting the messages in your client by date.
However, I maintain that a purely linear discussion model doesn't solve these problems: it just makes the content not exist in the first place. If you were to build even just a two-level system--links and posts; to contrast with a class web forum's four levels--which on HN would look like each article discussion being a linear chat, you could only have one of these conversations, as the interleaving would be crazy.