Hi, Pablo! These are great questions, and I just joined these forums to respond, after seeing your note in The Coral Project newsletter.
What was the first site you remember to have comments, or the first time you noticed comments had a real impact?
I think it would have to be, gasp, LiveJournal. I used LJ comments to not only communicate with my friends and respond to their entries, but we also used comments threads for role-playing games with different fandoms. (We were geeks.) This is probably when I first started using commenting frequently. I would get email notifications whenever I received a comment to a post, or a response to a comment I wrote. I also remember when comments could be edited, which was huuuuge.
When did news sites start to open up to comments?
Hrm, tougher question, because I never really engaged with news site comments. I would estimate the 2000s?
What other comment or community building experiences do you remember (forums, blog sites, BBS’s, reviews, etc)?
Forums/message boards for sure, as well as Internet Relay Chat (IRC), if that counts.
Can you think on anything you found as groundbreaking on comment UIs (likes, upvotes, etc)?
I think the upvote mechanism is pretty significant. I liken it to the highlighting feature for the Amazon Kindle; you see what people are talking about the most, and are engaging with the most, and that undoubtedly influences what you’re subsequently thinking about with regard to the topic.
I also believe the addition of avatars is significant, adding personality or a persona to comment and its user. Going back to LiveJournal, this was important for role-playing because we’d have multiple icons for characters and emotions.
I don’t think this is as significant, but some comments allowed for subject lines. I believe these are a smidge redundant and take up more space than needed.
Regardless: Long live comments! I’ve included some screenshots here from LiveJournal for your reference (edited to protect privacy of friends). Hopefully this helps! (PS: I took a stab at comment design over at Behance… would love to hear your feedback.)