Just hit this issue: .xbap (Avalon Web Browser Application) launches fine from VS, but crashes if you launch it from the file system. Reason? You probably have a .dll that you are using inside your application that you created as a "WinFX Custom Control Library" and in its .proj file, there is a <UICulture/>. However, the .proj file of the application itself, which you likely created from the "WinFX Web Browser Application" has no <UICulture/>. And things blow up.
This is a known issue with the November CTP -- the workaround is to remove the <UICulture/> from the .proj file of the control and recompile.