Anyway, I went digging through the specs, and one thing jumped out of the discussion on multi-user support:

Screen pinning: pin your screen so another user can access just that content without messing with your other stuff

This. This is fantastic for a specific use case that I've got. I have a Play Store All Access thing attached to my Gmail account. I'd like to share this with my daughter, without giving her full access to rummage around my email and all the rest of my stuff. If this means what I think it means, then I can "pin" Play Music to my account and let my daughter do that without having any of my other privileges. That's excellent.