A 'collapse thread' function would go a long way to eliminating the damage meta-threads and troll/flame-threads do to discussions. Yeah, I know there are extensions/userscripts that do it.

And yeah, I get the concern PG has about community fragmentation, but when I see a comment "leave HN" that has half a dozen replies under it, I really don't think I need to go down that thread of discussion any further.