I think that both the traffic and longevity suggestions were good and I would throw in in-bound linking as well, to see how well the directory is marketed. Maybe even just searching on the directory's name in the search engines versus checking the actual linking could also reveal information.
One last point that many people would probably skip over is activity. How many sites does the directory have listed, and more importantly is it growing and are the dead/inappropriate/bad sites being removed?
-- This message may have been cut off and the rest will only be shown to members. To become a member, click here --