Union Square Ventures' Fred Wilson wrote a post titled Social Layers and Social Intention. In it, he asked why the simple, 140-character maximum Twitter has succeeded, while the more ambitious FriendFeed ultimately failed to make it big. His answer?
Because FriendFeed was largely a social aggregator whereas Twitter is a service with specific social intent.
"Specific social intent". Yes, that's a great term. What focus does your site have? It's a powerful way to characterize the basis upon which software succeeds or fails. This comes from Fred Wilson, who has been right about things quite often with his firm's investments in Twitter, Zynga, Foursquare, Tumblr and others.