Upvoting is not for agreement. Upvote if you feel that the comment is well thought out, puts forward interesting points, or is otherwise worthy of the attention of fellow HNers. "Upvote" == "I want to see more comments like this".

Do not upvote just because you agree. I agree with the comment nominally, but I downvoted it, because it is a waste of space.

Simple test: if there were more comments like this, would HN be a better place? If yes, upvote. If no, downvote. If indifferent/not sure, move on to next comment.