I think the up and down arrows are perfect representations for what they do: "I want people to see this" vs "I don't want people to see this".

The problem is when someone believes that people shouldn't see any opinions that they disagree with. How do you stop them from downvoting substantive content? Personally I don't think changing the icons will help

I don't entirely agree. I think that upvote and a flag, placed elsewhere, would have a substantial difference.

That implies to me that it is meant to be used for when the rules are broken (like the flag feature as it exists currently). But non-substantive content isn't necessarily rule-breaking

HN/reddit moderation is extremely basic compared to the stuff Slashdot had 20 years ago.

We desperately need forum software with better forms of moderation.

"Basic" isn't necessarily "worse". I always found the voting/moderation on Slashdot to be very confusing and it made me not want to participate on the site.

That's interesting. I was a /. power user back in the day. Sometimes I think about implementing a modern take on slashcode. What about it did you find most confusing?

In my experience on HN, downvoted comments are almost exclusively low-effort, spammy, immature, etc. I'm not sure I've ever noticed an insightful dissenting comment downvoted here.

The [-] allows you to hide the most popular threads and dig for hidden gold. I do this often.