Hello,
I start looking at the code (build 22882) and I didn't see all related videos yet. So sorry if the question is discussed in some of them. Moreover it is the first time I see the extension method functionality. And I guess that having MVC in the middle make things worse...
But let's focus on the question... 
About the creation of extension methods, I found it extremely powerful but in large projects it doesn't seem easy to manage (Dll references and using statements takes another meaning) I don't understand very well the folder structure and the namespace naming convention regarding extension methods and how did you plan to further "extend" the solution.
1) Sometimes (for example StringExtensions) the static class containing the methods is defined under Kona.Infrastructure namespace but in other cases (for example UrlHelperExtensions) the class is defined under System.Web.Mvc. Additionally is a good practice to use System.Web.Mvc namespace for custom coding?
2) Naming classes: For StringExtensions or UrlHelper you use the type you are extending for naming the class. I found it very useful. But in other cases (DBExtensions or Gravatar) you don't use the type's name.
3) Some extensions are under Extensions folder but some are outside like Extensibilty\Modules\ModuleLoader
Thanks,
Eduard.