If they wanted to make an adjustment to the downvote karma threshold, I would suggest "require comment with down-vote" would be an interesting thing to test.
I would of course assume that less people downvoting + more specific complaints would lead to more moderator action on the more obnoxious/distracting posters, if that wasn't the case I wouldn't recommend the change either.
I'm aware this is how the majority of HN members use the down-vote feature, but the first time I've seen it presented as working-as-intended.
The fact that you're saying this makes me think we need more formal guidelines on the proper use of this feature. More often than not I see it claimed that down-voting should be an expression of "adds value" vs. "does not add value".
If I had to choose, I'd pick the latter.
Actually, that may not be a bad way of putting it. Comments that are blatantly false do not add value. Comments that get something wrong, but are otherwise high-quality, still add value. So this distinction may capture the nuance we want here. Let's think about it.
On a site like stackoverflow where the goal isn't discussion but answers to questions it makes sense to use a combination of mostly upvotes (for good material), lack of any votes (for mediocre material), and very occasional downvotes (for actively bad/outright incorrect answers). In that case it makes sense to engage with the poster and explain the problems, which can increase the potency of the downvote by providing context, while also helping to correct the original poster's misunderstanding.
On a site like HN that is mostly just opinion and commentary those patterns don't make sense. Most of the time comments that deserve downvoting are the sort that are either poisonous to productive conversation or just so monumentally wrong they need to go away. In that case engaging with the poster is precisely the wrong course, because doing so usually exacerbates the problem. Either by validating the behavior (if it's trolling) or by dumping a side-discussion into the larger conversation and polluting the SNR.
Your karma is proof of your ability to judge a comment without the need to refute it, whereas anyone with <501 karma should only be left the opposite?
I get that not all downvotes NEED a comment categorically but there's some meat on the "downvotes deserve comments" bones.
Also: A couple times I have accidentally hit the down arrow. There is no undo. How does this relate to comment quality? The only remedy is to add a "Sorry, I downvoted you accidentally" comment, which I've seen people do. Although it's good etiquette that's a relatively low-quality comment.