I like people who can do the following:
Given a controversial topic that the person has a strong opinion on, he/she can argue with people who hold an opposing viewpoint without resorting to logical fallacies or name-calling.
The person can summarize and repeat the opposing viewpoint in an objective manner that satisfies those who hold it.
The person can then effectively argue the merits of the opposing viewpoint to those who share his viewpoint.
I find that people who can do this are rare and valuable to have as friends and confidants.