[loup-vaillant]

Depends on the forum. HN emphasises that, by not notifying users that they had replies. Reddit on the other hand colours your mailbox in red so you're aware of replies without actively seeking them.

I don't know which foster the best quality discussions, but I feel the HN way is a bit impersonal.

[vidarh]

A trick I finally hit on for Reddit a couple of years ago was that when I start feeling a discussion does not feel fun or interesting anymore, I look away while I click on the inbox icon.

When I don't see the replies, they're easy to ignore.

At some point (I have no idea when), Reddit also added a "disable inbox replies" button to comments, so that you can prevent notifications on a comment by comment basis.

[Phenomenit]

Why is it set up like that on HN?

[loup-vaillant]

I have no idea. Perhaps they want to avoid discussions form derailing? Reddit routinely has long sub-threads, but they're hidden behind a link by default.