It is my understanding that one of the central purposes of the new top bar was to unite the sites so that there is a feeling of staying within the same master site (Stack Exchange) as you browse the different sites.
Something that I think would help with this experience would be to keep the top bar logged in even when you are on sites that you have not set up an account for. I think this new feature should not automatically create you an account on that site, but should use some sort of general Stack Exchange network account to keep the top bar features active. This way I can still see my notifications and my top sites even when I'm on a site that I don't have an account on.
How does this sound? Is this doable?
EDIT - Maybe a "join this site" link where the rep and badges are normally to indicate that you don't have an account on the site you're currently viewing?