The option to return from only sites popular on HN, blogroll, and the other "manage optics" settings are incredibly cool and useful, I could see myself using this just for that feature alone.
I take it you aren't showing dead threads? If you look at newer submissions you'll see people voting bad stuff to death. HN is insanely decent at community driven self moderation. Not to knock the mods who put a lot of work into the site of course, but I assume the community's own self-moderation helps some.
"Polluting HN" is more than just the vitriol thats flagged - there are plenty of self-promoting, downright wrong, or comments clearly unrelated to the posted article but comment author gets to segue to their hobby-horse (off-topic discussions are annoyingly frequent, IMO). HN is better than most, but its not immune to being gamed. Once there is a financial incentive for it, it will become more common - see how Twitter turned out after offering monetary incentives for engagements.
My comment directly answered parent comment on an issue pertaining to search engines. How is that a tangent or remotely off-topic? This meta-discussion, on the other hand...
I think HN could cope by weighting upvotes/downvotes/flags more heavily based on the age and reputation of the user taking that action. It was distribute moderation responsibility to users a bit more.