Simplest way to do Validation in WPF is usually implementing IDataErrorInfo interface, and do the validation in the indexer's getter. It turns out to be ugly and gets out of hand when your model / viewmodel gets a little larger.
ASP.NET MVC has great ajax integration with no doubt. Simply by using ajax forms you can use partial page rendering and there are also controller callbacks. One thing that will break if you switch to Ajax / JsonResult is the built-in Validation result (ModelErrors) rendering of the MVC engine.
Well, this is not actually my first code review, but this is supposed to be a copycat of 'Code Review WTF, Number N' series by Ayende Rahien.