I have the impression that this is mainly a point-and-click GUI for an implementation of a quarter of Common Lisp.

To translate their calculator tutorial (http://www.jetbrains.com/mps/docs/tutorial.html), in Common Lisp you would write a little macro "define-calculator" that then lets you write something like this:

    (define-calculator java-and-php-hours
      (input java-hours :label "Java hours")
      (input php-hours :label "PHP hours")
      (output (+ (* java-hours 10)
                 (* php-hours 5))
              :label "Total"))

It would be based on some library (e.g. common-qt) that provides GUI primitives.