It would make me feel constantly judged, arbitrarily even (because how would you ensure level-headed tagging?). It'd be as bad as downvoting, which is deliberately not possible on HN (with few exceptions).
I agree that it'd be dangerous to let anyone tag. People on YouTube usually mark anything they disagree with as spam. Hopefully it wouldn't be the same here though.
After reading your comment I realized I've seen downvoted comments but I can't downvote. Is there a karma requirement?
I just feel that part of the reason people descend to name-calling is because there's no feedback before they hit post. Nobody says "That's not a very good reply." Afterward, it takes someone motivated enough to call it out. In the meantime people see it and think "Lots of name calling on here." I think it's good that downvoted comments are still visible rather than removed so that people learn what is considered unacceptable.