This is my personal vote:
1. Good mixture of active posters - I think it keeps a forum balanced in discussion
2. Minimal amount of arguing; more peaceful/relaxed setting - I got sick of all the blood-shed
3. Cool website design - My forum has to be cool looking and FAST!!!! Everything in my life is fast, why not my forum?
4. Good variety of thread topics to talk about - I tend to gravitate to topics that interest me anyhow
5. Maximum amount of posting freedom - I consider myself a moderate "middle-of-the-road" kinda guy.... I personally don't need much freedom to be able to say what I need/want to say....