Check if you have an app.config in your Windows store project. If so, delete it.
This is a bug in VS 2012 or NuGet or both. Under certain circumstances, NuGet creates an app.config with assembly binding redirects, but Windows store apps can't have assembly binding redirects. The app fails to activate. This could also be a bug in Windows 8.
We haven't been able to put together a reliable repro for Microsoft. Here's the MS Connect issue I opened a while back.
https://connect.microsoft.com/VisualStudio/feedback/details/761276/nuget-incorrectly-adds-app-config-with-assembly-binding-redirects-to-windows-store-apps