Well what do you think a forum needs to have to be really called a forum, a place. Because I personally don't call everything forum that is ment to be a forum. Empty forums are not forums for me. For me the definition of a forum is a place where you can discuss and post on alot of topics and subjects, interact with the members, and much more ...

Thus for me the 5 things would be

1. A good team of admins and mods

2. content lots of it

3. A clear goal, purpose

4. original & interesting

5. seeing the site expanding and improving