Not all content is made equal. On average, I'd say there's more than a minute's worth of effort put into each minute of video posted to YouTube. Most of the feeds on live video sites are static cameras pointed at televisions, puppies, etc. There's some effort involved in setting these things up, but it's certainly less than 1:1 with the amount of footage being uploaded.

I'm not trying to trivialize justin.tv's accomplishment, I just don't think it's a particularly useful comparison.

That's actually a really interesting metric: the amount of effort per minute of video.

A quick mental runthrough of various video sites I've used suggests this would be an effective predictor of content quality.

Indeed. Another question worth noting is, how many people are watching? The ratio of consumers to producers is big -- the fact that YouTube has people to watch all the content produced on it is what's remarkable about it.