Well, if you want to mirror how GFY got started and how it got huge, you should focus on RECRUITING THE RIGHT PEOPLE. Just like in real life, forums work on INFLUENCE. If you recruit the right person, like a BONEPRONE or some other person who has a lot of friends in a particular social circle, their circle comes with them gradually and buzz builds. This is true of whether you're building a wm forum, a forex forum, a WOW forum, or whatever. Forums are SOCIAL so they mirror SOCIAL dynamics. It's all about influence.
So my suggestions are:
1) Find a niche where you know people with influence. How? Go to social networks and find people who have a lot of clout--example: Xeni Jardin of BB has a ton of clout. Give them a REASON for 'hanging out' at your forum Build an IM rolodex of people that fit this bill.
2) Give those people MOD powers to shape and sculpt the forum to their personality.
3) Seed the forum with strategically placed threads that lets the "forum celebrities" shine
4) Install VBSEO or other similar search engine optimization package for forums
5) Hit the rolodex carefully--updating them of what's going on.
Gradually it will build until you hit a Turning point and the forum takes on a life of its own.
Seems like too much work? Not really if you do it right. The latest forum to do this right is a mainstream forum that blew up fairly quickly due to the reputations of its initial members.
In forums, INFLUENCE is the only thing that matters..... unless your forum is really a microniche Adsense farm. In that case, I have another set of guidelines based on what I've observed
Good luck to you and NEVER EVER SURRENDER!