Building everything from an educational badging system (http://bjk5.com/post/2426884194/khan-academy-now-has-badges) to an improved practice UI based on student behavior (http://bjk5.com/post/2071068737/khan-academy-in-the-classroo...) to better individual student analytics (http://bjk5.com/post/2783333652/students-need-better-self-as...) to basic UX improvements all over the place.

(Sorry for self-promotional-looking links...they're legit sources of information for this question.)