This thread started by wmw member Webwork asks some thought provoking questions on the general subject of forum success. There are many, many questions, here are a few of them:

1. Is having a clearly defined purpose of the forum important? A vision. If the purpose is "for me to make money" what's the chance of that working?

2. Do most successful forums start with some preceding connection of a significant number of people? Say, readers of some informative person's opinions? (What was the groundwork for WebmasterWorld?)

3. An action plan for building critical mass? Step by step, thought out ahead.

Rogerd has some insightful answers and comments on the subject of forum success well worth reading:

I've posted a "mission statement" in mod forums to serve as a touchstone. Most corporate mission statements end up being quickly forgotten, but I refer back to the forum mission statement fairly often when discussing issues with mods: how to deal with member issues, whether a topic should be added, etc

It's interesting to note that as with directories, seo forums are popping up on an almost hourly basis. If you're thinking about online communities, this is a pretty good read.