The default behaviour of Gnome Shell for the (quite popular) multi-monitor setup was to move only the primary workspace when switching workspaces. That means that the second monitor and so the additional space was kind of wasted. That behaviour could be modified so that both monitor workspaces are switched (using ctrl-alt-up/down, for instance) by setting the
workspaces_only_on_primary gconf entry to false.
With Gnome 3.3/3.4, the gconf backend was apparently replaced by the gsettings backend and dconf. So now the new command reads:
gsettings set org.gnome.shell.overrides workspaces-only-on-primary false
Just a note to myself.