But how many users were there on usenet? The special thing about HN is that it keeps a quality of comments way above average without needing to be too elitist; I think that limiting the conversations (and thus the flame wars etc.) is a choice.
But to highlight new comments there are many different solutions already, take a look at the other comments :)
The special thing about HN is that it keeps a quality of comments way above average
That was in fact also a quality of Usenet, before it went out of fashion for unclear reasons.
It would be an interesting exercise to mirror (or even move) HN to a nntp-server. Personally I believe the signal/noise ratio would improve drastically.
But to highlight new comments there are many different solutions already, take a look at the other comments :)