The upvote/downvote arrows aren't for disagreement, they're for adds to the discussion/does not add to the discussion. That's the proper reddiquette at reddit as well. Unfortunately, it's generally ignored there, but Hacker News is usually better.
See: http://news.ycombinator.com/item?id=117171 http://news.ycombinator.com/item?id=392347
This whole discussion is getting rather off topic though. Does someone want to make a meta-topic for this, or should we leave it alone?