I watch Vihart's videos, and the comments are full of low signal. All of the high quality comments are from G+ from anecdotal evidence. Let's look at the top ones on her Dragon Doodle video:
"Ever wonder why fractals are called "fractals"? It's because they have "fractional dimension". This idea comes from noticing that when you scale a 2-dimensional object up by a factor of 2, its area increases by a factor of 4, but when you scale a 2-dimensional Sierpinski triangle up by 2, its area increases by a factor of 3. With a normal 2D object, if you scale it up by a factor of 3, you'd increase the area by a factor of 9, but when you scale a 2-dimensional Koch snowflake up by 3, its area increases by a factor of 4. So fractals don't behave like normal objects. They behave as if they have fractional dimension"
First non-G+ comment? "Can I marry your mind, please?"
Or this one: "hey, no knock on community colleges, studied at a couple myself I just feel like everyone's giving this girl way more credit than is due, but like I said, that's just me, and it has been made clear by everyone and their mother watching this vid that it is ONLY me. lol"
Or this one: "Yeah, you're totally a dick."
General consensus is, YouTube comments are, by and large, shit. I don't think you're going to convince many people that they're worth it because you can find a few videos that might have slightly higher signal.
Frankly, I would like to see "unification" of comments so when I'm looking at a video, I can see what people are saying about it from around the Web, rather than the same video linked to from 15 different blogs. For example, if some new tech video comes out, and is covered by HN, TheVerge, Engadget, Reddit, etc, I end up having to visit 4 or more sites to see what people's reaction to the video is, rather than having them collated in one place.