PG has posted on HN saying that downvoting for disagreement is fine.
On HN, it isn't as much a problem, from my experience, because the downvote button is masked for users with <~500 karma and I haven't seen many false positives.
On reddit, I was familiar with the saying -- "downvote isn't disagree button." Though, the current rediquette has removed that language. Compare [1] (old) v. [2] (current).
[1] https://www.reddit.com/wiki/reddiquette?v=08ebd986-6459-11e2... [2] https://www.reddit.com/wiki/reddiquette
Unfortunately, it is a serious problem. The moment you become a dissonant voice is the moment you're downvoted into unreadability by the choir. Make no mistake, many people here have enough karma points to downvote and they are quick to react.
[1] https://news.ycombinator.com/item?id=117171
edit: yeah, the downvote threshold system was bumped up ~1 year after PG's comment.