One thing that really annoys me about HN is how damn contrarian everyone always tries to be. Responses typically take the most absolute narrow and unforgiving interpretation of any point you are trying to make and subsequently use that to spin a counterpoint. Drives me batty.
I see two reasons: 1) If people agree with the idea they'll probably just nod their head, or upvote. If they disagree it is more likely they'll post something rather than just downvote (people feel the need to explain perhaps why they downvoted) 2) Geeks feel the need to say something contrarian to show how smart they are. It is a way to tell everyone they have a bigger intellectual penis than everyone else.
Also, to another of your parent's gripes - if someone only responds to a narrow point, it should be safe to assume they agree with the other, or broader, points, and that's why they didn't respond to those.
But silent agreement and loud disagreement does make it seem like everyone is mad at each other all the time.