the premier Virtual Education Journal showcasing the exciting work being done in virtual environments from Second Life to Minecraft to educational presentations to machinimas, it has something for everyone
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