Social Applications are usually best individualized per company
I find that every organization wants something different when it comes to social software. However, some common threads run throughout - the ability to tag content is important b/c helps to inform the user base about how content is really used. Commenting is great on publishing pages but the ability to add comments to documents is also really a great capability as most organizations struggle to get past using SharePoint as an online file cabinet.
I'd start by building the application outward from the individual person. Build a My Site which lets the person talk about themselves and expose information to the public. Next, make sure their destination site on the intranet can track the information they're interested in by allowing it to search across your sites (and site collections) for tags and activities of certain interesting individuals.