My understanding is that there were two changes:
1. General responsibility for authenticating to Google services has been moved to Chrome, and being logged into Google is equivalent to being logged into Chrome. If sync is set up, logging back into Google also fixes your sync session, if it was broken (common — the warning in the toolbar is easy to ignore).
2. Sync has been separated from login as a Chrome feature, so that you can log into Google services without syncing Chrome’s data.
Confusion between being logged into Google (the Chrome new tab page looks a heck of a lot like the Google home page) and being logged into Chrome was a real problem. I’m not suggesting that this is the right solution… or the wrong one; this is weird territory.
It raises a question for both users and browser vendors: What does it mean to be logged into a web browser?