With apologies to Senator Ted Stevens of Alaska: A community is not a place that you just dump people on. It's not a big truck.
A community is an organism, and one of the things that defines an organism is its specific arrangement of internal structures. If you stuck a person into a Blendtec blender, you'd end up with the same bill of materials in the end but lose the person.
People have skin, and cells have membranes: Organisms work by separating themselves from each other and the environment. They work by maintaining careful balances between volatile chemicals and controlling reactions through semipermeable barriers.
So, in communities, all relevance and life is lost if you just throw people and content together in a big soup. There need to be structures and semipermeable barriers to keep everything from combining all at once into the dull grey of stirred-up and melted Superman ice cream.
The trick, though, is to find a way to foster evolution in community growth. Maybe a little top-down seeding of structures, maybe a lot of bottom-up affordance to build neighborhoods.
It'd be really cool to figure out that trick where I work.