IMO The best way to encourage this is through better (default) notifications. Or, this solution, for a platform with (tens) of thousands of users like HN, seems to automatically bring the specter of flame war (and intrusion and what else ?)

Is there a middle ground? I don't know. Edit : Daily/Weeky email (=slower) updates ?

As for old threads getting bumped, I think this is here equivalent to reposting the link (or re-asking). The old thread is almost "dead", but this is how it works "in real life discussions". So maybe that is not so bad. This is (as a call it) a (fast) stream model, where there is no (direct) accumulation : it is more twitter than Stack Overflow, more blogs than Wikis.