Ask HN: What Kinds of Comments Should Be Upvoted?

We continue to see metadiscussion about comments and voting rules here on HN. Let's emphasize the positive by discussing what kinds of comments deserve upvotes as positive contributions to the community. Here are some suggestions:

1) Comments that link out to research sources to verify disputed facts, or that cite sources that aren't posted on the Web.

2) Comments that ask for sources to back up statements made in previous comments, which ask for comments in category 1.

3) Comments that respond to name-calling or ad hominem comments of the kind identified in pg's article "How to Disagree" by refuting the central point politely and thoughtfully, and not being drawn into the same tone as more argumentative and less informative comments (in other words, comments that raise the tone of a thread).

What other kinds of comments do you think should generally be upvoted? What kinds of comments are unexpected, pleasant surprises in how they contribute to the community?