Conditional content. Authors can change what content a lesson shows, depending on whether the student reading the lesson has done a particular exercise, what degree program the student is in, whether the student is colorblind, and a host of other variables.
Exercises. Students need hands-on tasks. Not just one or two, but a carefully designed task sequence.
Grading. The grading system makes personal formative feedback easy, fast, and consistent. One of Skilling's best things.
Feedback. Skilling makes large scale grading feasible. It automates everything except human judgement.
Questions. Skilling lessons ask students questions, as they read.
Characters. Characters are head shots of people expressing different emotions. I use them to represent students, taking the course alongside the real students.
Pauses. Pauses break up lessons into chunks. They implement the segmentation principle.