The general rule in groups with reasonably intelligent discussion and community moderation, once a community consensus is reached on a topic, is that:
Agreement with consensus, well articulated, will be voted up strongly
Disagreement with consensus, well articulated, will be voted up and start a lengthy discussion
Agreement with consensus, expressed poorly, will be voted up weakly or ignored
Disagreement with consensus, expressed poorly, will be voted down viciously
People who complain about groupthink are typically in the habit of doing #4 and then getting upset because they don’t get easy validation of their opinions the way people who agree inarticulately do.
As an example on LW, consider Annoyance, who does both #2 and #4 with some regularity and gets wildly varying comment scores because of it.
Hal, sorting by “popular” doesn’t seem to do what you think it does (what it actually does I can’t figure out). Sort by “Top” to get ordered by score.
Also, anecdotally, I can tell the difference between “agree” and “high quality” to some extent. I’ve up-voted several comments I disagreed with because I thought they presented their idea well.