Another point I would add are that brand matters a lot ... if you are asking people to pay for something were there are free offerings in the marketplace (even if the free ones are far worse) then people really need to believe in you to want to buy from you. Thus we offer a good number of things for free, including some tools & our blog.
The last thing I would say is that many private membership websites are quickly ghost towns that die. The hardest part is building up the initial critical mass. You can always raise rates later, but sometimes it helps to have lower rates off the start to buy that marketshare and help build momentum. For example, we charged only $50 a month for the first 100 customers & gave our ebook customers a free trial...these meant we had a vast active community right out of the gate. Then over time as we kept adding more value (more tools, bigger archives of information, etc.) we raised prices.
A sites like ExpertsExchange comes to mind. Also, there is another company which I recently became aware of that offers "social learning" college classes. There are no instructors, but instead a social network of tutors and mentors. They are gaining some ground. http://www.straighterline.com/ I have no affiliation with them, but recently read about them and sounds like an interesting idea.