Borrowing from the concept framework provided by the Wikipedia entry for Immersion, I think an ARG must have at least the characteristics of emotional/narrative immersion and cognitive/strategic immersion; basically connecting to a participant's interest and level of focus/engagement. From my perspective, the emotional/narrative component is key, because I don't think the act of just tackling a series of puzzles makes something an ARG.
And if we then add blended & augmented reality elements, then I think there's the opportunity to allow for sensory-motoric/tactical immersion as well as spatial immersion.
Wow, here's a gameplay clip that captures all three aspects of the immersion experience, sensory-motoric, cognitive & emotional; and it somehow even manages to throw in a bit of cognitive dissonance for extra measure... I think it's a great example of the impact creating these kinds of 'ah ha' moments can have and how these moments can distinguish an ARG as an engaging learning platform: http://www.youtube.com/watch?v=Y8AFiaWz7WI
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