Remember MTER framework (below)?
The topic, that very thing that your community is about, should be based upon one or more of these.
Typically organisations opt for things that are expensive or spend a lot of time on. However, if you look at the most popular communities on the internet, you'll notice that most of the world's largest forums aren't clearly based upon any topic at all.
Many of them are based upon having a distinctive personality. This is reflected in how they discuss themselves.
Reddit clearly falls into the distinctive personality category. It attracts an audience of generally young men drawn to the personalities reflected on the site.
4Chan goes for the shock-treatment. MetaFilter and Something Awful cultivate unique personalities based loosely around topical news and entertainment.
GAIA Online (8.5bn+ posts): "Welcome to Gaia's forums, where millions of members gather to discuss random stuff, make new friends, complain about life, argue about nothing, laugh at dumb pictures, discuss serious issues and/or curse like sailors"
None of these has a specific topic focus. All of them cultivate a very distinctive personality that appeals to different groups. Those personalities are representative of who we believe we are (or who we want to be).
We haven't explored the nature of cultivating distinctive personalities in communities. We haven't explored removing members for being too boring, for example, nor removing content because it's not funny/shocking enough. We haven't explored the other end of the spectrum neither, the serious debate about the big issues. If the market for frivolous is big, there should also be a market for the serious and substantive too.
The benefit of cultivating a distinctive personality is you don't narrow your audience to existing customers or people solely interested in a specific topic. Personalities transcend topics.
The most addictive communities out there today are those that have cultivated very distinct personalities.