Some people have a link from their G+ to their youtube, but it seems not to be there for everyone any more.
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).