There are three options - upvote, downvote, or do nothing.
If someone says something interesting/insightful/useful, upvote. If someone says something totally inane, downvote. If someone says something which seems reasonable, but with which you don't agree, either upvote or just don't vote.
Re:Tell HN: Disagree ≠ Downvote
by endlessvoid94 on Monday February 14, 2011 @09:56PM
(#2219921)
I was JUST thinking about this yesterday. I wrote a messy blog post about it, but the gist is that having conversations online isn't messy enough for real humans yet. Upvoting and downvoting being used as a "i agree" and "i disagree" is inevitable.
Part of HN's success is contributed to the simplicity of the interface. A two-choice voting system doesn't necessarily acurately display the user's intentions, but it promotes the most amount of involvement from them. If you were to go with a more complicated voting system, I would suggest going with what is outlined over in this thread: http://news.ycombinator.com/item?id=613071
1) "That was an insightful comment, and I agree with you" 2) "That was a reasonable comment, but I disagree with you" and 3) "You're an idiot"