http://news.ycombinator.com/item?id=117171
"I think it's ok to use the up and down arrows to express agreement. Obviously the uparrows aren't only for applauding politeness, so it seems reasonable that the downarrows aren't only for booing rudeness."
If you check the currently posted site guidelines,
http://ycombinator.com/newsguidelines.html
the only guidelines you see about downvoting occur right at the end:
"Resist complaining about being downmodded. It never does any good, and it makes boring reading.
"Please don't bait other users by inviting them to downmod you."
A thread opened by pg 466 days ago
http://news.ycombinator.com/item?id=2403696
asked, "How to stave off decline of HN?" He wrote, "The problem has several components: comments that are (a) mean and/or (b) dumb that (c) get massively upvoted."
So at that time, he thought that a significant subset of comments was getting too many upvotes (and, by implication, too few downvotes). That led to a software change such that users can no longer see the net comment karma totals of other user's comments. (You and I can still see the individual net karma totals for each of our own comments.)
To sum up, what I really like to do is upvote good comments. But there is no rule against downvoting a comment if it is mean (definitely not), nor is there a rule against downvoting a comment if it is dumb (the trick is perceiving whether or not a comment is dumb), and especially there is no rule against downvoting a comment if it is both mean and dumb. And if a comment is just a snarky remark that doesn't advance the discussion, or a lame attempt at a one-liner joke, or otherwise doesn't add value to the discussion, it doesn't harm the community to downvote such a comment without further reply.
(They're not really logical opposites and their use frequency is quite different.)
I'm happy to upvote more than downvote. I get the sense that some of the best stuff here on Hacker News still isn't upvoted enough.
Thats the way it should work, and is part of what makes HN different to say Reddit. In reality, its becoming increasingly common for people to downvote something simply because they disagree with it.
Edit: Would whoever downvoted this care to explain why? I'm genuinely interested.
It would be nice if Upvote meant "This is good quality (whether I agree with it or not" and downvote meant "this is low quality, and should not be here, but is not spam, nor flagable". But since PG has said (something like) downvote can be simple disagreement you're going to meet people who use it for exactly that.
Also, reddit is trying to move away from "downvote to disagree" - they have a lot of hover text saying this.
The reasons for clicking may not be symmetrical, but the actions when clicking are symmetrical. The points go down with the down arrow and the points go up with the up arrow. This is symmetry even if the reason (agree, disagree, quality, spam, etc.) for the increase and decline of the points is asymmetrical.
However that norm changed when people realized that was a bad idea.
The UI didn't change with the norm change.
At one point the norm was you could downvote to disagree if the comment had a score greater than 1. But now that comment scores are no longer displayed, that norm changed too.
Now the downarrow basically means spam/stupid/useless, etc. Actually for spam you also need to click on flag.