Can commenting on youtube videos (including signing into google+ etc) happen or is it not implemented? I'm on an ubuntu linux build
Took a while, but finally made it work: Open the menu of the onion icon and click "Preferences...". In the window that opens select the "Security settings" tab. Uncheck "Don't record browsing history ... mode)" and "Restrict third party cookies ... data". In the content tab of the options window, uncheck "Block pop-up windows". Now you're all set to login to google+ and leave a comment. I recommend starting a new Tor session (identitiy) for the purpose of commenting videos, after which you re-check all the unchecked boxes, log out of Google+ and start a new session.
You may sign in to google+ and youtube, and comment on youtube, however, you lose anonymity as soon as you do so. In addition, you will be marked as Tor user. Once somebody is marked as Tor user it becomes easier for adversaries to test the hypothesis of if it was you who visited a particular site or wrote particular comment even if you do not sign in, that is if adversaries choose to do so in some future point.