HN is good because of the quality of the submissions... which is still slightly questionable at times, and I've only been around here for a couple years, not long enough to assess whether it's dropped dramatically.
Average quality of comments drop with the quantity of users; that's something anyone who's run any number of web forums knows. Unfortunately, because the users also curate HN's content collectively, despite any weighting that might go on, a higher number of users leading to a lower quality of content will also lead to a lower quality of curation.
This is why Slashdot's article content varies with the quality of the editors, largely independent of the quality of comments. HN doesn't have that advantage, or any of the disadvantages that come with it. They're interesting foils in this case.