Only Start A Community For These Benefits
Brands usually have the wrong objectives for starting a community.
Most of the problems brands experience can be traced back to these initial objectives (and thus expectations) of what a community can/can't do.
Don't start a community of you want to reach new customers, increase brand awareness, undertake traditional marketing activities to or for anything short-term.
Only start a community of you want to increase loyalty, get invaluable feedback, find great recruiting opportunities, improve customer service or discover new sources of revenue/sales leads from existing audiences.
If you get these objectives right in the beginning, you save yourself a lot of trouble later. These objectives shape your entire approach to developing a community. The former focus on speed, reach and extracting value from members. The latter focuses on fostering relationships and adding vaue to members.
And, remember, the benefits of a community are derivitates of its success. You need to develop a successful community before you can reap the benefits.