What makes serious game motivational?
Not all games are fun. There have been many computer game failures simply because the games were not entertaining.
Papert claimed that activities are fun when they are challenging. Too much challenge results in frustration and too little in boredom. Challenge within one's zone of proximal development is invaluable because success in a challenging situation increases intrinsic motivation.
The built in game mechanisms that guide the user and give feedback provide scaffolding to assist the student to complete the tasks that may otherwise be beyond their ability. Students are most engaged when they can complete the tasks with the assistance of the built-in game mechanisms. The most important metric when choosing serious games is if they are challenging enough to make the students want to complete them, without being too difficult.
Enhancing student active engagement, which is actually a mix of motivation, interest and attention, is widely recognized as an important goal to be reached in learning activities. It is important that teachers and educators make use such educational means that can be more stimulating and motivating for the students and that are able to sustain and maintain their interest. Both personal attitudes and learning tools employed play a major role to foster motivation in learning activities.
More detailed in the paper "Motivation and engagement in computer-based learning tasks: investigating key contributing factors" by Michela Otta & Mauro Tavella.