Even though Nautilus will include a lot of the features already integrated into Nautilus Elementary, the two development teams will only collaborate on implementing undo / redo functionallty and Zeitgeist integration. The rest of new features will evolve separately in Nautilus and Nautilus Elementary. This includes breadcrumbs, revamped sidebar, zoom slider and the editable toolbar.
It's nice to see the Nautilus Elementary developers will finally be working with the upstream Nautilus but its unfortunate we won't see some more Nautilus Elementary features in the default Nautilus - features which many of us find to be incredibly useful (and eye-candy too); at least not they way they are implemented into NE.
But the great news is Nautilus will be getting Zeitgeist integration upstream. It will be very interesting to see how this is going to be implemented - Zeitgeist can do some really amazing stuff!
Zeitgeist is an event aggregation framework for the user's activities and notifications (files opened, websites visited, conversations hold with other people, etc.) and makes the relevant information available to other applications.
Here is a somewhat old video with Nautilus Elementary + Zeitgeist (via ammonkey):
You can read more about this and the rationale behind not adopting more Nautilus Elementary features into Nautilus, on the Nautilus mailing list.
