What sort of traffic should you have regularly prior to opening a new forum. The new vbulletin announcement got me thinking in that general direction again. I'm thinking that averaging 5000 uniques/day is a good starting point. How much traffic did you have before you started your successful forum.
I had nothing. My site is my forum. I kind of did things a bit backwards, but at the same time my forum was designed as a centralized area for some friends I met via Warcraft 3. 5000 uniques sounds like a good number to me. Though retaining forum members that engage in somewhat meaningful conversation is another story.
I also had no traffic. But the forum was started in a great niche where there was no competition (when it comes to forums), and people were eager to jump on and post.