You need something unique that no other forum has, for your free members, and subscription members (meaning they paid to be a member like a V.I.P. group or something).
I'd give my idea but...then it wouldn't be unique when I get a forum running in the future.
To me, it isn't just content...but also the modifications and such too, and how the staff is. You can have a nice site with unique stuff but the staff are fucking assholes and no one wants to deal with that. So in most terms,
Jason's correct. Unique content is important too. Local bands that are getting big or are signed are good stuff to post for music. (Asking Alexandria, Of Mice and Men, etc.) something that isn't "too well known, but well known" will help since there will be a certain amount of people looking for it...rather than 0 or 1 person.
Movies go the same way, same with games, etc. You need to find something (whether you buy it or find it somewhere else online) that little to no other site has.
Last of all, be creative...visit as many sites within your site genre and see what they all have in common and what they don't have in common. Mix and match, provide EVERYTHING they provide PLUS MORE. This requires something thinking but will help greatly in increasing your site popularity, traffic, user return rate, etc.