For instance, if you simply hate the Shell and would like to use Gnome 3 with Metacity, then you simply can't do that!, as simple as that. But the rest assured, it looks promising though. A brand new theme interface with better looking graphics... in a way Gnome had to introduce something to compete with Microsoft Visa or 7's Aero I suppose, even though we have Compiz, still it is not an official Gnome project. So, they wanted something of their own.
Anyway, if you have installed Gnome 3 beta, then Smooth Inset is a beautiful looking theme that uses the new Gnome Shell. Go to this Smooth inset devianart page for more information on downloading and installing it.