Drives me batty.
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.
Everyone thinks of the karma/reputation feedback mechanism in terms of how it promotes civil discourse, but in this case it's fairly remarkable to think of how it could be biasing the conversation toward a negative tone. Maybe it's usually civil negativity, but it's negative nonetheless. I can see how people would tire of this eventually.
I wonder if there'd be a simple way of limiting the bias? Slashdot's modpoint classes comes to mind, but then that opens a whole host of other problems...
It may make #2 worse though. People love to argue against the majority, especially geeks, especially on the internet.
Thought that's rarely the case. What usually happens is someone will read a clever article, get upset that they didn't come up with it and choose a narrow interpretation or a minor nitpick to attack the piece, thereby making themselves feel better.
More often than not, I hit back instead of submit.
Sorry, I just couldn't resist.
Edit:
To actually add something to the conversation (and sadly, to be just a bit contrarian), I hope nobody reads this as "oh darn, I better not take an opposing view for fear of being voted down as contrarian." I actually do like reading respectfully contrarian responses. Not the childish ones you describe, but the ones which have a bit of thought behind them. I think a better term is "devil's advocate."
Please people, keep doing that, at least.
While in certain "communities" this maybe is driven by the profession it self (oh, CS and all its true/false 0/1 domains ;), very seldom "cooperative discussion" is presented as something beneficial... just check most newspaper and tv reports... we get controversy presented as the way to go while the world is clearly much more complex as to be grasped in simple black/white true/false terms.
For single humans tending to such patterns in thinking certain medical conditions are associated with... in our societies looks like we embrace it much more thoughtlessly. All has to be a controversy (Parties, Companies, People)
Sorry. :)
Also, anyone that bothers to publicly point out to you a misspelling is someone with too much idle time and not enough decency. One must hope for more cunning and aggressive frenemies so you may be judged accordingly.
I find that productive discussions do grow out of conversations with people who can note both what they agree with and what they don't. I've actually learned quite a bit from this.