I agree with Skinner - a sense of community keeps people coming back and genuinely engaged.
My other 'must have' is a proper focus - I see lots of start-up forums that have a zillion different categories... basically covering everything the author is interested in - but why would anyone join? You need a narrow subject matter to start with, whether it be web authoring, motorbikes or guitars. When you've got an established forum community then it's fine to add on less obviously connected 'community interest' categories, such as football.
