As paradox as that may sound, in a sense the opposite of open-mindedness is needed. There needs to be more social pressure on people to keep discussions civil and to stay intellectually honest. The problem right now is that people advance argument for rhetorical purposes only, in order to "win" instead of solving problems together. Social pressure to change that requires people to be less tolerant towards intellectually dishonest people.
Of course, I agree with you that open-mindedness is important when dealing with intellectually honest people.
The paradox is "pressure to keep civil" to you may not be the same to me. Numorous times trying to call out someone not civil will put you in the not civil category. That's why systems like HN aren't necessarily encouraging of open discourse. Many don't want to get judged, and judging is all many do, downvoting based on rhetoric and neuance (or lack thereof). And, the achilles heel is the downvote as the not-civil vote, except there is no discourse because you can't reply to a downvote.
Of course, I agree with you that open-mindedness is important when dealing with intellectually honest people.