One point I'd raise about this is that I think it's fine to be able to indicate progress, be it just to keep the user looking at something while the page is loading, but I don't like how it seems to duplicate the function of the other progress indicators the browser has. Seeing the rise of client-side web apps, a browser API to show progress directly through the browser's chrome rather than custom UI patters would probably be great and prevent a lot of the fragmentation that things like this might cause. Plus, the less-savvy users might come to expect to see this bar everywhere just because they see it on YouTube, which I think is not a healthy thing for the web in general.