What do you think about the idea of senior members? In my forum there are a few members that have been selected for the rank and gain some privileges.
Interesting question. I'd like to hear some thoughts as well. I've asked this question before (http://www.adminaddict.net/forum/member-staff-management/your-first-members-2765/), but we have many more members now who can give us a wider range of opinions.
I generally make a premium member group on all my forums with additional permissions. It doesn't cost much, usually $3-$5 through PayPal for life. It gives things like close own threads, give more rep / day, more PM's, those kinds of things. Usually I give the first 25 members or so free premium memberships. That's usually as far as I go though.