One source advices to reinstall the gnome-system-tools package to restore the menu.
Another source advices to reinstall the gnome-main-menu and gnome-main-menu-devel (not required) packages, and adding the nautilus-gnome-main-menu package.
Still another source advices :
I find ~/.gconf and ~/.gconfd and move them to gconf and gconfd. I
copy /root/.gconf and /root/.gconfd to ~, change the owner, relog. I
do see the top panel!
The thread
Ubuntu 11.10 Upgrade: Missing Your System Menu (Preferences & Administration) Launchers?,
although for Ubuntu rather than Debian, makes interesting reading.
You might try the several solutions mentioned in it.
Other people admit giving up on this and going with KDE or Unity.