Ask What are the key features of a successful membership site?

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I'm building a membership site and want it to succeed. I see many sites offering exclusive content but I'm not sure what else is important. Some sites have forums, while others focus on courses or discounts. I'm trying to figure out what makes a membership site appealing and effective. I assume some users must know the essential elements that attract and retain members. What are the key features of a successful membership site?