What rules would you put in place?
How would you manage it?
What would the forum be about?
For me, first thing I would do is have no rules whatsoever in my forum; all deicisions on behavior are decided by the king of the website. I would be the king of the website, having control over every other member and post. In addition, I would have some very trusted members become semi-kings, who have similar power to me, but I'm still the strongest.
We would decide on things as they come, jusging carefully what's a troll post, what's a spam post, or what's a flame post as we see it. We would also keep tally and record of what members say what, so that we have an idea of how offensive their posts are. Also in order to join the site you must submit an essay (a short one) concerning something about the site. And if your suspended or banned, you may submit an apology essay any time later in the future and be un-banned if you proved your behavior is better.
The site would be about either Magic the Gathering and other related activities, or a typology site that branches off into various other forms of psycho-analytical personality stuff from various other sources (Like MBTI, JCF, the MtG colors, and D&D alignment comparisons).