Enhancements are available for Revit 2014 for all Autodesk subscription customers. Extensions that are missing from subscription center will be available in Autodesk Exchange Apps store. Users can login to Autodesk subscription center to download them.
Unfortunately, these extensions are not available for public download yet. Though Autodesk has put these applications in Exchange Apps store, they are only available for subscription customers for now.
Earlier, Autodesk released extensions for AutoCAD and Inventor for free; it is only for Revit extensions that this ‘subscriber only’ type of licensing is provided. It might be that with an intension to promote Exchange Apps store, Autodesk is making all these extensions available only for subscribers.
Apart from extensions that need to be purchased, there are many free extensions with great features that are available in the Exchange Apps store.