The main change in the WinJS SDK is that an issue with WACK certification has been fixed.
As for XAML/C#, there was an issue with app that used previous version of the SDK crashing after update. It has been fixed in this release.
You can get the updated version of the WinJS SDK from our download page.
XAML/C# SDK is available through nuget.
Please consider that both SDKs are still in beta and make sure that you’ve tested your app with the SDK thoroughly before submitting it to the Store. In case you encounter any issues, we would appreciate if you could report them to firstname.lastname@example.org