Quality unique articles are worth alot.
Quality consistent users
Active admin
Unique / semi Unique topic
Good domain name
Good board software and skin
Thats what i used in most forums i opened, most forums don't get too busy, but some that do take off really take off, specially if the topic is very much in demand. I don't concentrate on the American market to much anymore, i run forums with topics like bollywood, of politics forum of middle east, asia, or an European country, defence forums, just recently i opened an emulator forum which i am working on.
Btw, one other point is always keep forums fast, a fast loading forum is a good healthy forum, i run all my sites 4 servers.
Search the web for ideas, i wouldn't like to call it stealing but there are alot of forums out there which have some really nice ideas which you may want to implement on your board too, it maybe topic, hack, design, contents.
Sorry but i don't generally disclose my properties on forums i do business in unless i specifically want to advertise or sell them.