The exercises for students target specific skills and help build student confidence and let them know if they are on track. So much better than worksheets as students get instant feedback on their answers and so they cannot cement wrong procedures. It's also great you can assign students a very specific exercise or video that they are struggling on to review. Students like that they earn points and can upgrade their avatars as well.

I love how Khan Academy keeps track of student work and percentages. Also, the simplicity of the whole design/program. Very user friendly. Easily assign work to the whole group or specific students.

Learning pace.. broad topic areas.. categorization