I might be mistaken but I get the sense that, especially to older members, it's not just "declining quality of discussion" (uninformed/irrelevant, discompassionate, outright rude, inflammatory comments) that is threatening the value of the site. There may actually be a less tangible/measurable quality, that of the early "feel" of the community, that was of great value to those members. It might have come from the particular set of voices that made up the bulk of comments in the early days, or the relative obscurity of the site. While discussion quality can be improved, I doubt some members can ever feel fully satisfied, without the old feel.

Also, there are many innovations in comment systems. Gawker's Kinja system is quite good at surfacing or organizing interesting comments, among other things. Post segmentation and comment filtering are techniques used by Slashdot and Reddit, which dampen the impact of growth on community "feel". But, despite the massive growth, there is strong aversion to incorporating any deep changes here.

I perceive two specific challenges: prominent voices might actually want a feeling that is (or appears to be) impossible to restore, and there's general apprehension about messing too much with the magic formula that brought HN to the point it's at today. With the influx of users, this appears to create an identity crisis for the site.

I wish I could suggest solutions along with these observations.

Edit: I do think it would be useful to understand what members of varying karma levels subjectively/qualitatively perceive to be the values and valuable traits of the site.