Downvoting should not be used for simple disagreement.That's nowhere in the Hacker News Guidelines.
http://ycombinator.com/newsguidelines.html
You are, of course, entitled to your opinion about how to use downvoting, but other users here are entitled to theirs.
Here is what pg said about the matter 1280 days ago:
http://news.ycombinator.com/item?id=117171
"I think it's ok to use the up and down arrows to express agreement. Obviously the uparrows aren't only for applauding politeness, so it seems reasonable that the downarrows aren't only for booing rudeness."
There have been refinements in the site software since then, notably the change of a few months ago to hide individual comment karma scores from readers other than the comment's individual author, but there has never been a statement from site management that downvotes must necessarily be explained. Sometimes the best way to respond to a comment that is not thoughtful and well informed is to downvote it.
When pg wrote his post earlier this year "Ask HN: How to stave off decline of HN?"
http://news.ycombinator.com/item?id=2403696
He wrote, "The problem has several components: comments that are (a) mean and/or (b) dumb that (c) get massively upvoted." By implication, some comments don't deserve any upvotes, and may indeed deserve downvotes without further subcomments.