Three design themes I see becoming more important over this app maturation period are Centering, Faces, and White-Alpha.
The apps I find easiest to use are balanced. ScrollViews opting for centered content instead of the traditional left to right are cleaner. This is primarily handled by "cards", a centered rect for object content. This also emphasizes pacing, displaying no more than 3 to 4 objects to digest per view plus with load times decreasing users can fly through the cards as quickly as they please.
Identity has become an app imperative. Products last because users see them as the place to publish in their respective space. Our identity is tied to our content and our profile photo provides deep detail into who we are. Photos are eliminating text. Once you read my quick bio, my profile image acts as a referencer and button to my identity on the platform.
I say white-alpha everything. The app should fall into the background and the content should come forward. Emphasizing/Deemphasizing particular data depends on the app but in all cases I see off-white and off-black backgrounds/layers the most useful touch interfaces. This is because I see all successful apps as utilities, therefore the only thing in an app that matters to me are the users and the content.
In each case, I am concentrating on bringing the content forward in order to give users a consistent and immersive experience cross-platform. Whether we are moving to larger screens on tablets/tables/walls or to 3D-in-the-air displays, giving users simple ways to interact with the people and content of said platform (especially with integrated gestures like touch-expands, drag to trash, and swipe aways, which i see increasing in popularity) is king.
In order to last, apps need to be platform ready utilities. Integrating these themes can help.