Hi there coo
I have been on a number of forums spreading over a few years now, i think what makes a good forum work is one that has good administrators, ones that are laid back enough to allow what can be heated topics, but ensuring respect is kept at all times. One thing i notice regarding this forum is that Michael pops on and off here a lot during the day and i get the feeling he looks at everything being posted.
A good forum is about respect, freedom, good subject and very good administrators.
One idea is to go to a lot of forums on line and see what they are doing right and wrong, some forums develope cliques which i feel is never a good thing as it makes new people feel very left out in the cold and not very welcoming.
Plus if there is a good forum out there thats already doing the same field as your interested in, you may ask yourself why do you actually want to set up one when something is doing the same job.
I personally would not set up a forum like this one as i feel its doing a great job with amazing people in its own right.
Though i know other forums can be a bit stuffy and there is room for improvement
You also have to ask yourself what do you actually gain from one, what is your goal, how much time are you commiting to one, how much true help will you have on board, not people saying they will help and never showing up. Look at your aims, your goals and work back from that to find the steps you need to take to achieve it.
I have a friend who has her own forum and she is very dedicated to it, but i also am very aware that it absorbs a huge amount of her time as well, its almost a full time commitment without any financial gain but finacial lost due to paying servers etc.
Make sure you look at every aspect and then decide.