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I'm sure many of us would agree that Hacker News discussions are generally of high quality. Comments are usually thought-provoking, arguments solidly supported. What I have noticed, though, is that discussion on HN only proceeds for about one or two days in most cases. Yet, I think threads would carry on longer (and wouldn't degrade too much) if participants got notification of new responses.
I feel quite discouraged to post anything on an HN thread that is older than 48 hours -- I suspect extremely few people would read it, so I'd rather not spend my time. If I knew that HN article discussions tended to last longer, I'd be more encouraged to participate. As it stands, I can't always keep up with the latest news, and sometimes read news items 3 to 5 days after they are posted, but by then it's too late to participate.
If anyone could come up with an HN subscription service, I, for one, would be very grateful. :)