Sometimes I disagree and someone else has already provided the new information I was planning to.
Sometimes I have a problem more with tone than with content -- someone was right, but being a jerk. I'll both downvote them and upvote someone else who was right and more appropriately civil.
Sometimes a comment is simply pointless. Someone posted a meme, a lame joke, or a comment that has nothing to do with the topic at hand (like "this" or "totally correct"). Downvoting is a nice shorthand for "this comment doesn't add anything to the discussion".
It's the distinction between "this isn't the content I'd prefer to see at HN" and "this comment is abusive or inappropriate for the venue". Karma doesn't mean much, and getting the occasional off-topic or vacuous comment downvoted is just a soft form of negative reinforcement.
In the first case, then simply upvote the comment you agree with. Remember that when you downvote a comment, you're essentially downvoting the entire discussion associated with that comment as well, affecting its visibility to others. Don't downvote substantive comments just because you disagree with them, it robs HN of quality content.
If everyone downvotes to disagree, the net effect is to reinforce the majority view and create an echo chamber. Use the downvote to enforce community standards, not community opinion. Many people have a hard time understanding the distinction, but it's an important one.
Sometimes I have a problem more with tone than with content -- someone was right, but being a jerk. I'll both downvote them and upvote someone else who was right and more appropriately civil.
Sometimes a comment is simply pointless. Someone posted a meme, a lame joke, or a comment that has nothing to do with the topic at hand (like "this" or "totally correct"). Downvoting is a nice shorthand for "this comment doesn't add anything to the discussion".