I used to be a frequent poster at a board that set up a private area for members that contributed financially. It actually worked really well, and served as a bit of a 'thank you' to those that donated money. On the upside, people in the private area were more open about their real lives, and several friendships and even a marriage resulted from the camaraderie there.
The downside is that the moderators have a tougher job. All the normal TOS rules applied of course, but you could tell the moderation was a little more relaxed. That's a good thing, but it was also a slippery slope, and a tightrope the mods needed to walk. The other problem is that there's a tendency for things to get a little cliquish, and the moderators had to make sure people weren't talking trash about folks on the outside of the private area. That was a big no-no.
But overall I really liked it. I know that's not what we're talking about here, but I thought I'd throw it out there.