Xfce is a lightweight display manager available in debian wheezy out of the box. It looks like a perfect alternative to a default Gnome3 to run on old hardware. Also in Gnome3 (as well as in Ubuntu 12.04/12.10), I could not find an easy way to prevent the notebook from sleeping when I close the lid (Power settings apply only to the user settings, but not globally). Xfce does not have this problem, and my notebook can continue running with the lid closed.
The only annoying problem is that the touchpad is not activated by default. Also the Xfce mouse settings do not cover touchpad behavior. But the problem is actually solved quite easily. Under root, create a file “/usr/share/X11/xorg.conf.d/70-synaptics_custom.conf”, with the following content:
Section "InputClass" Identifier "touchpad catchall" Driver "synaptics" MatchIsTouchpad "on" MatchDevicePath "/dev/input/event*" Option "TapButton1" "1" Option "VertEdgeScroll" "1" Option "HorizEdgeScroll" "1" EndSection
Reboot the PC, and voila, tapping and scrolling works everywhere.