I have never done a directory but from knowing people that have them it can be a tough market to crack as everyone wants one-way, do-follow back links.
I guess if you can work out some kind of strategy that will benefit not just you but the link submitters too then you could very well be onto a winner.
Where my directory differs from most is I seldom even provide links from
mine, but still get 100+ new listings a day without even trying to gain any.
I occassionaly get people emailing asking where their link is or complaining
about not having one, to which my reply is,
"It is a business directory not a web directory or link farm"