Seriously?
Spam, trolls, racists, and flamers were routinely "thumbed up" in comments. And then we have the comments consisting entirely of "exact quote from the video you just watched" which typically have hundreds of thumbs up as well, and add absolutely nothing to any discussion on the video itself. I don't think I'm alone in saying the SNR of Youtube comments was low to the point that I never sought out to read them any more. And when I did, it was more of a "lets see how bad the train-wreck is" mentality.
Moreover, did Google/Youtube said they pushed the change due to quality of comments ?
Stupid comments = good stuff? :)
The second are videos from my friends, or select channels I subscribe to. These usually have a small viewer base (<10000 views), and in the comments one can actually have a useful discussion.
Of course now many of these small feeds say 'Comments are disabled for this video'. I assume it's because the channel owner does not want to link their google+ account, or some technical fuckup - it seems unlikely they'd all change their settings, but I guess it's possible.
Not that Usenet didn't (and still does) have its problems, but I feel you're right about the quality of the discourse in some of my old favorites. However, I've personally always attributed that to the higher barrier of entry than the medium itself. See: "the September that never ended".
I frequently use the https://youtube.com/tv layout after seeing it on my Samsung Smart TV