Why is this focused update so important for developers?
- OAuth2 support: Now your apps can be deployed to the ArcGIS Marketplace, since ArcGIS Online Authentication can now be used to authorize users in your QML or Qt/C++ apps.
- Support for DirectX on Windows: Now your apps can run through Remote Desktop and other virtualized environments.
You can find a lot more information about this update in the release notes.
There is also an important blog about migrating 10.2.5 Windows apps to 10.2.6 here: http://blogs.esri.com/esri/arcgis/2015/06/04/arcgis-runtime-sdk-for-qt-migrating-10-2-5-windows-apps...
Get out there and rock your cross-platform apps!