You can’t get the efficient discussion UI of usenet clients anywhere, except maybe on mailing lists with a specialized mail client like Mutt. Usenets clients were peak discussion UI. Reddit is a far, far cry from it.
It relies on keyboard navigation, threading, and per-message tracking of read/unread status, and on subject lines. The threaded view with one line per message allows a quick overview over which parts of a discussion one has already read or which are new, or are still unread from the last time. You usually have a split screen where one part of the screen shows that thread tree and the other shows the currently focused message. Focusing a message marks it as read. Common key bindings are pressing Tab to jump to the next unread message, and Space to page-down through the contents of messages. This makes it very efficient to catch up on a discussion (or really, to catch up on all ongoing discussion over all subscribed newsgroups) even after a few days (or longer). And if you reply some time later, you can still expect other participants to pick up your reply and easily see its context.
Replies don’t get lost because the discussion has moved on or the thread is a day old, like they do on HN and Reddit. This enables long-running, structured discussions. There are more usability aspects related to the text editor used, such as editor commands for handling quotes when replying. There are of course the usual(?) forum features of being able to ignore (filter out) specific users, and other features like automatically highlighting/ranking/filtering messages based on other criteria. But the main usability benefits come from per-message read/unread tracking, keyboard navigation, and the compact thread representation.
Note how on HN and Reddit you have threading, but because each message is displayed inline, you see less of a thread at once. You also see only one topic (thread) at once, whereas in a Usenet client you see all recent or currently ongoing threads in the same view as the intra-thread structure. This causes a more shared experience of subscribers of a newsgroup being aware of the current ongoing threads (over days and weeks) than on Reddit and HN. And most importantly, when returning to a discussion thread, on Reddit and HN you don’t see which messages you have already read and which you haven’t, and thus threads die out after a short time, and longer-form discussions are not practical. The experience is profoundly different.
