If you're not getting down voted a reasonable proportion of the time, you're too timid a thinker (or your comments avoid all controversy and stick to technical matters).
Downvotes are ideally for low-value content. If your viewpoint is contrary to popular opinion, the onus is on you to explain your reasoning persuasively enough to not come off as a simple contrarian.
Downvotes are not a reward for thinking independently. They are a penalty for sloppy thinking or overly presumptious output. They are also the only way I can think of for fighting populist, short-circuit comments that troll for upvotes - by encouraging actual argumentation.
Certainly, some people stop, think, disagree, and downvote. I believe that this is the case when the original comment is sloppy, and generally warrants a reply. In some cases, especially various kinds of politics, the replies are omitted presumably because of fatigue. Such people have already replied to the argument enough times in their life to not want to do it again. That could be interpreted as a failure of domain research on the part of the original commenter, but I don't like to blame people for not being "well-read".
It's quite possible for the majority of people to vote based on agreement, and yet have the overall score of the comment be determined by those who vote by quality. Of the ones who vote by agreement, some will agree and some will disagree, and they end up cancelling each other out. The remaining comment score is made up of those who voted because it was a well-reasoned argument.
Reddit has the "Controversial" tab for this, though HN unfortunately doesn't make a distinction between a comment with 50 upvotes and 40 downvotes vs. one with 10 upvotes and 0 downvotes.
It's possible for this to happen, but I don't think there are that many hot-button issues where the two sides are exactly balanced. More commonly there will be at least a slight skew in one direction or another, which gets magnified in absolute score the more people there are who vote based on agreement. For example, if there's a 60/40 opinion skew among people who vote based on agreement on a particular topic, and 20 people vote that way, you already have an 8-point rating gap between the popular and unpopular comments (+12/-8 versus +8/-12). On really high-traffic topics, where 100 people might up/downvote based on agreement, you only need a tiny skew from 50/50 for the votes to pile up in one direction or another.
I think in practice that often swamps the quality-based voting, especially on high-traffic articles: you sometimes see extremely short and not particularly insightful comments that express a popular sentiment up at +40 or more.
up-promote vs. down-demote: affecting order-of-presentation and karma
left-agree vs. right-disagress: a mini-poll per comment, with no censorious effect on presentation or numerical-reputational effect on karma
Is there somewhere to read up on all the rules of HN? I figured out a few things on my own (like that you have to have 200 rep to downvote), but once in a while I come across something I don't understand (like, it seems that you can't downvote immediate children of your comments?).
This kind of thing also tends to change. I think I got my downvoting privileges at 50 or so. Like words after a long night of drinking, voting has flowed a lot mre frequently of late. Pg usually modifies the caps to compensate, but they aren't usually announced. I noticed, for example, that /leaders is no longer in the top bar. I think that was a while ago, but just an example of how the site changes.
Here is pg's announcement: http://news.ycombinator.com/item?id=1173801
In the same discussion, in response to http://news.ycombinator.com/item?id=1174132, pg moved the leader link to the Lists page and had the Ask HN on top bar.
I make plenty of comments on controversial topics, often in opposition to mainstream opinion. I get lots of critical responses, but I only rarely see downvotes, and have only one comment that I know of that's below 1 (deservedly so; it was snarky and non-contributory). I'm neither timid nor controversy-avoiding; I just present my controversial ideas in a way that's engaging and enlightening.