At the moment I'm having to pretty much do all the editorial work on my site. I have 54 total categories in my forum directory and I'd like to have at least 5 - 10 sites in each before I start resting a bit. I'm pretty much having to do all the work myself at the moment. Like you said no one wants to submit their site to an empty directory so it is the admin's responsibility to get those categories filled. Once there is a decent amount of quality content in the then and only then will people have a desire to list start listing their own sites. You want people to feel like they are missing out if they don't submit their site. Same with a forum really.