Logging into youtube will, by default unless you know about it and turn it off, associate every video you view with your G+ profile for your gmail account, so every person you correspond with can look up your video history.
The other really frustrating thing is when you log out of Gmail, you are not actually logged out. You will be presented by a pseudo-looking login screen, which is really just an authentication screen to get back to Gmail.
What really happens is they keep you kind of logged in to collect as much data on your searches, videos, etc. The way to actually log out is:
- Click logout in Gmail
- Click "Sign in with a different account" (!)
- Click "Remove"
- Click the X next to your name.
- Click Done
It feels like they are trying to trick you into staying logged in by making the logout process extremely obscure and difficult, and I really hate it when I'm being blatantly tricked like that. With existence of evercookies (http://samy.pl/evercookie/) I am somewhat skeptical that even that obscure logout process actually logged me out.
This is pretty much the strongest reason why I don't use the GMail UI anymore (there are others, but this alone would have made me stop).
It appears to have changed in the last few months, and it didn't show full history, so I'll take it back slightly, but you can still see other Youtube users favourites/subscribed: https://www.youtube.com/user/whoever
Some people have a link from their G+ to their youtube, but it seems not to be there for everyone any more.
