What makes a great online programming course?

I'm designing an online course to teach programmers a new language (i.e. Scala).

If you could build the perfect online experience - what would you DO or NOT DO?

A few thoughts to ponder about: - recorded video vs interactive HTML interface with audio - test driven exercises - linear vs non-linear - style check exercises - slide-driven vs. exercise-driven - visualisations (e.g. inheritance) - unique visual style (e.g. 8-bit, green board) - community - badge-system - odd/different vs plain/serious - theme - work-related vs fun examples - ...