Put the user first. Provide something of compelling value.

• Branded experience (for existing organizations)
• Exclusivity/privacy
• Domain-specific features (e.g. Facebook will never support sharing Elgg plugins like the Community site!).
• High-quality content (you'll need good writers who are passionate about the subject).

You need to ask yourself if your value proposition is powerful enough to lure people away from easier, more reputable, more reliable alternatives. Why should they discuss the topic on your site vs Facebook/Quora/Twitter?

And don't forget the hours & hours you will have to spend constantly improving the site and dealing with trivialities thrown your way by people who really should have their IQ boosted to double figures or their access to a computer permanently deleted. ;)