Tim Cook just announced the biggest SDK for iOS since the launch of the app store. Apps in the app store can now offer services to other applications. Better yet, finally, third party apps can now add widgets to notification center.
If ESPN wants to, for example, it can release an app with widget notifications for sports scores right in your notification screen. New Safari extensions, like Pinterest, lets you pull in images from the web and post them to Pinterest right away. Obviously this can extend to all different sorts of apps and extensions.
System-wide third-party keyboards are coming, too.