Theme is a very important factor when joining forums . And of course genre plays the main part in joining a forum , you won't join one in which you don't have any interest .
Activity is not that much important , since every forum starts out with a very few posts and members , it's with time that it grows gradually .
As Raptor said, theme is a very important factor when it comes down to joining a forum. Everyone has there own taste and we can't help what they like and dislike. Obviously the genre of the forum plays a massive role in the community who would want to join a debating forum when it is based around games? Activity levels don't bother me that much a forum with 30+ active members and a few hundred posts all interesting and all members post at a stable pace then I am happy, so I do take joining forums into consideration.
