How about, here and on reddit, being able to mark threads you're interested in, and having a page where you can see those sorted by last reply. On HN, make that page refresh every 15 or 60 minutes or whatever. Heck, once every 24 hours would be enough... sometimes I just want to talk about the things that interest me, with the people that are interested in them. I would love to be able to think on something for a few days, or to familiarize myself with a subject before mouthing off.
As they are now, reddit and HN are require you to be there when something is "current", and that's ultimately not that much better than TV. Yeah, sometimes you can get something out of it, but it's nowhere near as useful or deep. Even worse, there is a tendency to get semi- or unrelated stuff one out when something slightly similar is discussed, instead of putting things exactly where they "belong", where they add to a useful corpus. [which is exactly what I'm doing right now, and am doing too often.. but I honestly would prefer the alternative which doesn't exist yet]
Just think about it, we have practically infinite storage, there's certainly plenty of expertise in reach of HN -- yet discussions get constantly restrained because our attention is limited. But it's limited because all we get are these flat lists, X items per page, next to no means to curate or organize anything. Imagine how much worse it would be without people who sometimes link to old but very relevant and insightful stories or individual comments. I'm very grateful to them, but to take them for granted, to rely on them for "structure" is totally stone age to me. Give us tools, give us transparency, let us figure out things even if they might be hard - stop trying to hand hold people you might underestimate! We don't need you as much as you need us.