Official WD answer:
Dear Juan,
Thank you for your reply.
I received a reply back from engineering. It appears that YouTube will no longer allow non-browser based players to play HD content. This is accordance with their rules and regulation that we have to adhere to. I apologize but you will not be able to play HD content from YouTube on the WD TV Live.
Sincerely,
Leonard B.
Western Digital Service and Support
http://support.wdc.com