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