I've never had to deal with framework version problems, as our software is for internal use only, but I can imagine figuring out what to target would be painful

I've never had to deal with framework version problems, as our software is for internal use only, but I can imagine figuring out what to target would be painful. You don't need to have a separate designer to take advantage of XAML, I've written several applications doing both XAML and C# myself. It does make it much easier to have a non-programmer work on the UI. Metro and WinRT have .NET bindings, and most of the samples I've seen are written in C#. It prohibits use of System.Windows.Forms for the same reason that you can't use the Win32 UI libraries, but the rest of the framework is available. Also, the Metro UI authoring is done using XAML. None of this is to say that I love everything Microsoft does. I'm sick of their sandbox, and the projects I do for fun are all Linux / HTML or Android-based.