Engaging mediums encourage thought
An engaging medium is one which brings the reader into active conversation with the material. It is a medium which encourages the reader to think, make, or act, beyond mindlessly consuming material.
It is difficult however to design engaging mediums. This could be because engagement takes effort on the part of the participant, or perhaps it is because we don't know how to design engaging mediums well. It is why many games have a tutorial, and some of the best designed games teach you naturally through gameplay. Games are inherently engaging mediums but only within the bounded world of the game.
Some research and progress towards engaging mediums:
In opposition with: Immersive mediums replace thought
Beware: Addictive mediums