What's an "auto Youtube login"?

Google tried to get users to link their gmail accounts and youtube accounts. Thus, if someone sent you a youtube link in gmail and you clicked it - you would be logged into youtube already/automatically so that they can track your viewing behavior.

I'm sure they could track you without linking through two accounts together. They also track all link clicks from within gmail.

Not saying the tracking isn't bad, just that the account linking had more to do with unifying the Google experience than tracking.

Actually, that was the whole point. They were not allowed to due to their separate service agreements/privacy policies.

Then, early in Q1 2012, presumably as part of the whole G+ ecosystem push, they unified/changed their Privacy Policy terms across all (most?) services which users also had to accept.

This actually sounds pretty convenient.