Coding it is easy, getting traction with users is the hard part. I'm honestly tempted to build one but I dread building it and having it be a ghost town because getting people to adopt sites like this is tough.
The trick seems to be keeping it targeted towards one unique and tight nit community. Then opening the doors for those users to branch out. Like if you had a "subreddit" for an MMORPG where community is already a key attribute.