Apart from watching out for offensive posts etc do you also expect your mods to start topics? I have recently asked mine to do so, not had much response about it and was wondering if this was an out of the ordinary thing to ask for?
I have a list of duties for mods (actually it extends to all forum leaders). It includes:
Spam Cop - get rid of the spam
Nurturing your forum - help it to grow, maintaining the rules
Editing posts - removing the "oh-oh's"
Deleting posts - as needed
Also, I have a Code of Conduct for mods that talks about their conduct, confidentiality, and forum duties.
IMO, nurturing forums is part of a mod's job, too. That not only means policing, but being there at regular intervals, answering questions as they can, helping to find answers, and if there is no activity happening in a forum, they can start threads to get some activity/growth.