Cool, didn't know CORS was implemented so widely, and its true about WebWorkers. But Chrome doesn't support <device> yet, or DRM. (I only mention DRM since the Youtube team says its one reason they can't fully move off Flash).
I thought the only reason YouTube hasn't isn't fully supporting HTML5 was ads. Afaik, YouTube doesn't use any DRM whatsoever.
Apparently they use it for movies and other proprietary content:
Webcams and microphones can be accessed via the HTML5 <device> API, cross-domain requests can be done with HTTP Cross-Origin Resource Sharing (CORS), which is arguably much nicer than Flash's cross-origin policy XML file solution, and multithreading can be done with web workers.