Comment 1 for bug 801464

I think I've got more informations.
It happens also to me, it's pretty annoying, but if you start with root account gnome-shell DOES start.

If you do startx from a non-root it sends tou to a classic gnome desktop.
If you launch a terminal and try to launch gnome-shell it says
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$ gnome-shell
failed to create drawable

(gnome-shell:6684): Clutter-CRITICAL **: Unable to initialize Clutter: Unable to select the newly created GLX context
Errore del window manager: Unable to initialize Clutter.
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I created a new user account, just to see if it was a personal configuration problem (that root doesn't have) and it doesn''t work exactly the same way.