When a discussion is interesting enough for me to want to follow it, I will search the page for "minutes ago" to find the newest posts. It might be nice if new posts were highlighted for some amount of time.
I have a user script for that [1]. If you visited a story's comments before, it will highlight new comments and can auto-collapse threads without new comments. Otherwise, it gives you a slider you can use to highlight the X most recent comments.
[1] https://greasyfork.org/en/scripts/18066-hn-comment-trees