From the new trailer it appears that the Mario-head planet acts kinda like a hub-- you can still run around without needing to do anything in the Observatory, but you need to step in front of the ship's wheel in order to access the worlds.
Playing through Mario Galaxy again, I'm a bit thankful for this change, as it's tiresome to go from one edge of the observatory to another over and over.
Thinking about it, I can see why the Observatory could be seen as a bland hub-- it didn't really go with the new mechanics the game offered. I mean, Peach's Castle was built to accompany the fact that moving Mario around in a third dimension was fun in itself, and Delfino Plaza was built for the use of FLUDD. The only notable thing in the Observatory that had relevancy to what the game offered was the Planet of Trials, where you could play with the gravity. Otherwise it was a glorified Peach's Castle, but there was not a lot of substance, just wide open spaces. Galaxy 2's hub-map hybrid seems to correct this problem.