I've always thought that downvoting was for things that didn't contribute, and for things that were technically wrong (including logical fallacies and similar). However, disagreeing on opinion doesn't strike me as an area (on HN or otherwise) where downvoting makes sense.

So downvoting "1TB of data can easily be uploaded over a 20Mbps connection" (Takes 4.8 days, versus a 20 MegaBytes ps connection, which takes ~14 hours and while not easy, is at least more feasible) -- should be ok. But a comment with correction would normally be better...

Downvoting someone for saying that they prefer working in Eclipse (just because I prefer vim) doesn't seem very useful?

I don't think that the line between "I disagree with your opinion" and "I think this is incorrect" is always clear, particularly when new theories or analysis is being floated. As examples:

If I state that "chocolate is better than vanilla", and you disagree with me, it's not really that you think I am incorrect; you simply just disagree with me.

However if I state that I think "[country] will do [something] in Crimea", then you might disagree with me because you do think that I am incorrect. However in that case, because my statement was speculative, there isn't a strong sense of "objectively correct or incorrect".

I think that most 'disagreements' in online conversations are closer to the second than the first.

Downvoting someone saying they prefer some text editor is useful because those conversations are often very dull.

Downvoting the incorrect post is valid but as you say it would be more useful to provide a correction post.