I'm on quite a few forums (fora?) with my various hobbies, interests etc. and find the biggest difference is getting used to the software each one uses and as I'm a prolific photographer, the methods of dealing with images from direct uploading to posts, through forum hosted to having to link to an external image hosting service, Photobucket ruined a lot of threads when they started charging a few years ago.
The admin/moderators also play a big part in how successful a forum is, worst type I've seen is where admin are always right and if you dare to argue you get banned! and the cliquey type where new members are flamed so leave.
I moderate on a forum that started out as a single subject, but now has nearly 35k members that post about such a wide range of interests that a google search on a lot of technical subjects will give a hit to the forum.