I've deleted my G+ account and I'm still kind of confused over what works or not. It's even designed so that you can no longer save clips to a list of yours (including "Favorites" or "Watch later") from the video clip page itself -- this brings up the "Join Google+" page.
But! Since your lists and their contents are still accessible, it made me wonder...
And it was shown that if you click on the upper left "hamburger" icon, scroll to the "Playlists" section, pick a playlist... Well, then the list is shown along with a fully functioning "Add video clip" button, which adds a video clip to the selected list on your disconnected account. Uuugh. I can only imagine it is to push users (back) to Google+...
So, AFAIK, you get to keep your YouTube account which can still store which clips you like and dislike, you can save them to default or custom playlists after jumping through some hoops, but you can't comment, your channel seems to be gone, and you can't upload new clips. All those "can'ts" bring up the "Join Google+" page for me now.
So in essence, if you are a creator, you seem to need a Google+ account. If you are a consumer and OK with not commenting, you don't need Google+.