You can't downmod submissions. And comment downmods already act as a filtering device by graying out the text of unpopular comments. I doubt many HN regulars would say that the site today suffers from a lack of comment downmods. Those who would choose these hypothetical filtering options would be choosing to passively consume HN as readers rather than as participants. Those who wished to remain active participants would be able to do so. This happens already, where a minority of "leaders" drive most participation and a silent majority consumes the site passively. The only difference would be that you'd be giving tools to the silent majority to make their experience more compelling by automating the filtering that they already do manually.