Some great points. I think it should be added that running a forum is a lot of work. Moderating your forum and keeping spammers at bay probably uses up 75% of your time once it gets going.

I've run a forum on a particular brand of digital camera for the last couple of years and it's been hard going keeping people interested but it's gradually growing with over 500 members now, sadly that's not 500 active members.

I have also made several long term members into moderators and admins which is a big help. Only do this though when you are sure they will contribute and not take advantage of their position.