P.S. I find the best way to find previous HN threads that I remember is a site-restricted Google search with a few keywords, e.g.,
http://www.google.com/search?q=site%3Anews.ycombinator.com+p...
Way better ~ http://searchyc.com/previously+submitted?sort=by_date
Separately, would it be useful if HN had a sort presentation that listed posts/threads recently commented upon -- perhaps further weighted by post score -- as opposed to just recent comments (where the quantity combined with flatness of presentation quickly overwhelms me)? Of course, one would have trouble finding the new comment within the old thread -- particularly active ones -- unless there was further utility added to locate it within the thread of conversation of such a post. (If it was in response to a parent comment, it would not sort to the top.)
At which point, the current presentation paradigm for the site is starting to change significantly. Representing both a lot of work for PG, and an open-ended question as to the effects upon the community dynamics.
So... perhaps we are better off with what we have. If a page/topic is brought up again after after a significant period of time (more than a few days), it is handled as a new discussion. With thoughtful people like Rider... perhaps citing older posts/threads for further reference.