Using a fresh Debian testing netinstall iso, and manually installing Gnome 3 instead of XFCE leads into the problem that the login screen does not show (and neither does any user action on the top menu).
# apt-get install aptitude tasksel # tasksel install gnome-desktop --new-install
The error log looks like
# less /var/log/gdm3/:0-greeter.log message = '"GDBus.Error:org.freedesktop.DBus.Error.Service Unknown: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name org.freedesktop.ConsoleKit was not provided by any .service files"' fileName = '"/usr/share/gnome-shell/js/gdm/loginDialog.js"'
Googling the error leads into some older bugs, but nothing special (but one which made me check if consolekit is installed). The solution is simple – the ‘consolekit’ package is missing, a slipped dependency somehow.
Install it, and reboot.
# apt-get install consolekit # reboot