Only this extension feature alone in iOS 8 will change everything. All the cloud storages you choose always available in any app. So for example when I video edit in iMovie I can pick images and videos either locally from the iPad, or from any cloud storage I use. See screenshot below.
Just brilliant. It will also make it not so necessary to have so much local storage available. You can store all your media files and other files in the cloud instead and always have them available in any app, without leaving the app. It will be very useful for me that uses my iOS devices to create content with.