What the other posters have said (in their snide little way) is true.. communities don't build for nothing, they are attracted to the site because something sparks their interest. Although it's true that sites such as MacRumors reach a point where their popularity could sustain itself beyond content, it was the quality of the content that brought people here in the first place.
You need to think of something that hasn't been thought of already, implement it quickly, and do it better than anything else, and MAYBE you can have a community like this one.