I have a low 5-digit UID on slashdot, and registered well after I started reading, so I was sort of in the second wave there. The discourse quality fell off and leveled out, but my experience is that low digit UIDs generally contribute at their historic high quality levels.
In my opinion, a low UID filter on slashdot would be far more effective than the mod/meta-mod/complain about mods and meta-mods response this system created.
I don't know how this might translate to HN; "Filter by age of account?" What if by default you only showed comments and posts from accounts older than two years? (This would disqualify me, by the way, so maybe one year..) :)
People could select to see comments from younger accounts if they liked, or perhaps very well seasoned accounts (two years on the site) could promote up comments to general visibility. This would be patently unfair, in that new awesome commenters would not get to contribute to the discourse immediately, but it would almost certainly immediately increase the quality of discourse.
Slashdot's system has some echos of this, but they made it far, far too easy to become a moderator as they wanted to spread the work out. They also I think created a mental expectation with the '-1' score which created a trolling subculture, that is, reading slashdot at -1 is a cesspit, and so nobody does it, hence people use it as a cesspit, hence.. What I'm suggesting is making it the strong default to do the equivalent of reading slashdot at say 4 and higher, but using age of account as the heuristic.
This would be a reasonably easy chrome extension, but obviously easier to do on your side. If you demo this yourself, I'd be curious to hear if you think the quality is improved.
In real life, we might call this an apprentice system, that is, you have to sit in the back, and only occasionally get your answers while you prove you wish to belong and that you deserve to belong. It's a known effective way of maintaining quality of members in a desirable group, from religious to musical to .. I imagine secret societies?