The application is really good and implements most of the GTD concepts to the T. However I feel GTD as system is not that comprehensive in "Do" part. It organizes the stuff but when it comes to actual doing, it simply presents a list of things to do.
However I am highly impressed by Kanban workflow and Pomodoro techniques.

I feel, in Doit application if you can add a feature like - "Kanban View" where all tasks become cards and can be moved around in the lanes to the closure, it will be great feature. User should be able to configure status change for each of these swim lanes that will get directly applied to the task.

Secondly, when "In-Progress", you can allow the user ti start Pomodoro timer.