I'm not sure if I've just drank too much Koolaid, but I am not surprised by this youtube change. It's been evident to me from the early says of G+ that Google wanted a unified login system. As a Google Apps user, it was so obvious that everything was broken about the login system. Now Larry is in charge, and he wants all products speaking the same language. Google first unified the design language to get you used to the fact that everything is one Google. Now, slowly all of your accounts are being unified.