In my mind, it has always been segmented. Yahoo, now Socialcast was a technical oriented volunteer group that you had to know someone to join. The cost for continued membership was participation. Every so often, the list would be culled for the non participants who probably didn't even realize they were kicked out. Yes, we did have some people black balled (Robert Woods comes to mind, as well as one in Oregon by Deb Thomas) but in recent times this hasn't been an issue. And as we have had recent discussions on it, I thought it was decided that new members would just be vetted by the four of us.
The 90Minds website was developed as a pay to play and has grown from that base (due mainly to Wayne and Mark). I agree that it should remain a pay to play site. I like Wayne's idea on the tier structure and I agree with Jim regarding goals and how to achieve them. I don't think revenue generation is a primary goal for 90Minds but it certainly should be a carrot/stick for driving participation. We might even stratify the fees based on level of participation. The more you participate with blogs and articles, the less you pay.
How to do that is the $64K question.
I am up for a conference call some evening to flesh this out more.