You could also think about having lists that were only visible when they had active tasks. This would be very handy for any of my RTM meta data checklist smartlists.

Obviously RTM is different now than it was when this idea was posted. Now, all you have to do is look at the list of Lists and see how many tasks are in each one. The empty ones have no number; very easy to see.

Ranbarton's idea has also been implemented already. You can choose to show only lists that have tasks.