People join a community for one reason, but participate for another.
You join a writers’ community to improve your writing. You need to interact with fellow writers to improve your writing. In doing so, they become your peers. You begin to care about their opinions of you and your work. Your thirst for recognition, influence and friendship within this groups keeps you coming back.
From the outside, your community needs to offer a clear benefit. Self-improvement works well. This community will help you become a better photographer, or kickboxer, or marketer. Being entertaining is good too. Join and get the latest movie, music and entertainment news.
But the best communities, once you’re inside, force you to interact to achieve that benefit. This is where entertainment communities so often stumble against self-improvement communities. This is why relationships and contributions always trump the best content.
More importantly, the best communities make it impossible not to interact. They force you to invest an idea, opinion, rating or criticism into the community. You might not even have a choice, other people might begin rating you the moment you’re in.