Oh, something important to add. If you restrictively license the content of your forum, you might in the worst case make it difficult or impossible to migrate your forum to a new platform. Don’t do that!
(There’s a social aspect too, although it might be minor. If your forum terms say that a person’s contributions become the property of the forum, that will put some people off. But you need something: you don’t want users who leave to be able to insist that all their posts should be removed. This is a different problem to the topic here, but it shows that terms are important.)