I've tried to put my finger on what made certain G+ discussions just tick, and it's complex and subtle.
Much of it came down to having a good host for the discussion, and among the best was the site's chief architect, Yonatan Zunger. He was well-connected within G+ and at Google (obviously), but also had a diverse set of interests, tolerated a wide range of opinion and interactions, but not without limits. (Similar in many regards to HN's moderation philosophy.)
Threads were anchored by the initial post or link, and Yonatan often wrote a fairly substantial piece himself, rather than just throwing up links or a brief tweet-length bit.
Discussions were limited in how many comments could be appended, I believe the limit was 500. Presentation was unthreaded flat, with most recent comments at the end of the thread ... but that was also what was presented by default (the full thread could be expanded if desired).
And "recent" participants (I believe this was the most recent 100 comments, could have been more) would get a notification if there were new comments. This could (and did) occur even years after the initial post, and occasionally old posts would come back to life.
Participants would receive a notification, and could view and respond to recent comments within the Notifications pane itself, rather than having to separately go to the post itself. This is unlike many other systems I've seen, including Diaspora*, which is otherwise quite similar but has a very cumbersome (and slow) notifications -> posts interaction.
Key to keeping this from becoming a magnet for spam or tedious arguments was the fact that the host could moderate (and remove) low-value, distracting, or inappropriate replies. Doing so would effectively remove notifications generated by such comments. Again, Yonatan was diligent in this --- over several years I saw a very small handful of low-value responses, and usually they had been removed in the few seconds between my own seeing them and writing Yonatan to alert him of something untoward.
The other element driving this was an effective search tool. G+ went through numerous iterations, some lacking search entirely and some with a very half-assed search, but for its final few years, there was actually an effective and useful search which could turn up interesting prior discussions. (Search is another feature Diaspora* is entirely lacking.)
As regards HN, several elements that made G+ so vibrant in this regard are lacking. There are no notifications, individual members don't host threads in the same way as G+ did, S/N is worse than G+ at its best (and that was by no means the typical experience).
For HN, the best way to resurrect an old thread is probably to either link it (or a comment) directly, or to re-share the original article/URL and reference earlier discussion. That would have to be more than just the "previous discussion" comments which are typically made.
I'll often reference my own earlier comments (or occasionally posts elsewhere) in responses, and have a few bugbears I'll raise again. (Those ... risk becoming tedious, I try to be mindful of that.)
There are a few themes I've raised multiple times and developed somewhat in the course of multiple discussions, two examples that come to mind are the prospect of trans-oceanic trains (using submerged floating tunnels), and the possibility of a tax- or ISP-brokered scheme of universal content syndication as an alternative to subscriptions and ad-supported media, both of which I've developed over several years.