I've noticed recently that for almost every submission that hits the top page, the top-rated comment is a skeptical response to the thesis of the link content or the poster's argument.
Go ahead, pick any 5 of the front page posts right now and see if I'm right or not.
In general, this is a good thing as taking the skeptical position forces a minimum degree of critical thinking, which makes that comment more useful. And it's an excellent form of filtering out the large amount of BS we come across everyday.
But I certainly would like to see more top comments that agree with what the poster said, or expands on their argument.
(Eagerly awaiting the top comment to this post !!)
Because there isn't a downvote button on stories, posting a comment that disagrees with the premise of the story has become the de facto "downvote" button.
It's easy karma to get, all you have to do is disagree with the sentiment of the post with barely more detail than "no, I disagree".
Then, when people come into the comments to argue that they don't agree, they see that there is already a comment that says essentially what they want to say so they upvote it.
The newest type of default comment that we're starting to see is the "disarming comment". On reddit this showed up as pun threads. I'm not sure exactly the form they will ultimately take here, but they're usually pretty highly rated as well. In any contentious thread there will be supporters by way of upvotes, detractors by way of the critical / skeptical comments, and people who attempt to make peace with comments that are either light-hearted or attempt to be conciliatory.
It's not just an HN thing though. Any community has these sorts of behaviors. It's just that there isn't a default way for someone to register their disagreement in the same numbers of ways there are for them to register their agreements; especially since comments that do nothing more than agree whole-heartedly with the submission and add nothing new to the conversation tend to stagnate or get downvoted.