I see people posting horrendous stuff with their real names all the time on youtube. I don't think anonymity is what causes these horrible comments: it's more the fact that the internet creates a "barrier" between two people. It's easier to say something with conviction in text than it is in person, even if you're typing it under your real name.
Edit: I guess I need to clarify that my friend was a perfect gentleman to the guy. Very non threatening. Not in the same road-rage way the other guy had been on the road. The situation ended very peacefully and the other guy apologized. This was not one of those "I'll throw your dog into traffic" situations. I can't imagine why confronting a bully like that would be looked down upon but I guess HN is fickle that way.
I'm not sure why Youtube even bothered allowing text comments in the first place, let alone why they didn't go to video-response only years ago.
So a YT comment stream takes on a party line vibe and (IMO) there's not much point taking the time to write something thoughtful.
Mainly when I have commented is on videos on niche subjects when I'm reasonably sure the video poster/artist/producer will see it.