Depends on way too many factors to give you a solid yes or no answer...
In your case a webmaster forum that focuses on just directory owners is too specific considering they can just go on any webmaster forum and discuss directory ownership.
But in the case of say a forum built just for webmasters of adult sites that might be different because none of the major webmaster forums havea section just for adult webmasters.
Niche is very good in communities. But, yes, it is important how you select your niche. You want to be specific enough that you can focus your marketing. However, you don't want to be so specific that the market is too narrow to allow your forum to become successful.
There are too many forums in the cyber space now. New forums got to be revolutionary. Perhaps, maybe charge a fee to become a member, then give part of the money back as commissions to members for inviting people.
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