@Paul:do you even know what is Silverlight and WPF ? no you dont.
in my opinion , HTML5 is a mistake. the W3C should have fixed some issues with their previous standards before thinking about HTML5.
Now there is a huge problem with silverlight in my opinion. There are too much differences between silverlight and WPF.furthermore XAML is too complicated for a client developper , compared to MXML and Flash. But the solution is not to make Silverlight a subset. the solution would have been better if silverlight was a framework on top WPF, so you can limit yourself to silverlight for rapid web dev or go down to a more complicated level if you want to go further.
you can use .cs , .js or .vb files to developp classes in WebMatrix so that is not a big deal. using .cshtml make sense because you can developp custom helpers too , without having to write a lot of code. Razor engine will parse the html code inside your @helper in the APP_CODE so you dont have to use string builders or htmlstring to write out html.
you decide the default theme folder ,so no issue here .
now for APP_ ... it is usual naming convention in ASP.NET so why change it ?
All the things you can do in C# , VB , Jscript ASP.NET , you can do it in WebPages framework.
well , i think it could manage xml/json files that describe different translations for a user interface.you could imagine browser prefered langage detection and management, a set of methods to deal with it , etc ...Anyway a lot can be done on this matter...