Your community should make it possible for people to do what they want to do. Or easier to do what they already are doing.
Communities that try to change or create habits usually fail.
Facebook made it easier to talk to your friends. Flickr made it easier to share pictures. Twitter makes it easier to follow what your friends are doing and thinking. We were doing this already, the communities made it better.
Cramster connects thousands of students to teachers. Global warming communities coordinates the efforts of activists worldwide. Commania lets community professionals share and discuss their best advice.
When you talk about mission, sense of purpose and benefits you’re really talking about what people are doing (that you can improve) or what people want to do (which you can make possible).