A lot has been written in the press and on blogs about the unwillingness of people to pay for content on the internet. The examples that are most frequently used are the big national newspapers who have tried subscription, but in most cases failed.
The reason they have failed is because there are excellent free alternative sources of information for just about everything that appears in today's nationals. 
And this provides a big part of the answer to the question 'who should start a membership website?'
Anyone who can provide unique information that is not freely available on the internet.
Uniqueness can be achieved through exclusivity, informed opinion or the bringing together of lots of information in one place.
Membership websites can cover just about any subject. The key to success is identifying and focusing on a niche within a broad category.
Paid content sites can be run by individuals, companies, government organisations, charities and associations. Indeed, this includes anybody or any organisation that wants to charge for access to their knowledge, information and expertise.
The main groups are:
1)      Individuals who are passionate about a subject.
It could be a hobby (e.g., fly fishing), expertise gained at work (e.g. running a restaurant), or experience gained whilst pursuing a lifestyle (e.g. buying a house in Florida).
Just about any subject can be turned into a membership website. It is especially attractive to people who are already recognised as experts in their field, such as authors, TV presenters or journalists.
2)      Existing publishers who wish to make their publications more compelling and profitable.
Traditional print publications with an existing subscriber base can easily be moved online and, in doing so, most of their costs, including printing, distribution, postage, payment processing and fulfilment, are stripped out. These could be business newsletters, consumer magazines or local newspapers.
3)      Existing businesses that wish to generate an additional revenue stream.
Most companies have information that many people would pay to get access to. This could be an accountancy firm writing about how a small business can save taxes, a travel agent advising independent travellers or a building company teaching people about how to manage a home build project.
4)      Societies and associations who already have a membership base and are looking for a better way of servicing these members.
A membership website enables a lot of information to be shared in a time-efficient way with a lot of people. It also allows members to communicate with each other and enables the website owner to see what information their members are reading. This is perfect for associations and societies.
5)      Sports clubs and fan clubs can grow rapidly and build loyalty through a membership website.
One of the greatest challenges that these clubs face is providing their members with regular information in a cost-effective way. There is also great benefit to the club and its members by getting people interacting with each other and sharing information and views. This builds loyalty to both the club and the website.
6)      Charities who need to share information with their supporters and donors to keep funds coming into their organisation.
The internet provides the perfect medium for both sharing information and collecting donations. This can all be done via a subscription website.
In general, any subject area that could attract a group of people who are willing and able to pay a regular subscription can become a successful membership website.