Being infected by the joindiaspora project (
https://joindiaspora.com), I like the idea that users can have friends from different domains where webmasters or admins are using the same software without signing up on that particular site. This means that I can see the activity stream of a friend, send private messages to my friend & comment on his status updates without ever leaving my site eventhough my friend is on a different domain.
This feature is like magic & I would think that such a software could grow strong very rapidly because users benefit from it a lot. Just think about how many times you had to sign up at different vbulletin sites because you wanted to get involved in a particular topic.