Taking up the OP's side because it's interesting...

Some kinds of groupthink are bad (like making an anti-vaccine subreddit that only ever shows articles about vaccine-related industrial accidents, instead of a vaccine subreddit that shows people a truly representative sampling of vaccine knowledge), but other kinds are arguably good (not seeing 4chan nasty stuff on /r/reallycutekittensandpuppies.) Only some group content coordination is filter bubbling, while other times it's filtering.

Yeah, this. A lot of good stuff submitted to HN never gets noticed because there's not much incentive to go through new submissions. It's basically volunteer work, so you get volunteer-quality results. The caliber of volunteer in HN is higher than most sites, but a lot of good stuff still falls through the cracks (like most of the stuff I submit ;-)