Right, this is a fair point, and what I think G+ is banking on. The abstraction and additional complexity is clearly there. On the one hand, the modelling part

Right, this is a fair point, and what I think G+ is banking on. The abstraction and additional complexity is clearly there. On the one hand, the modelling part of the complexity is a wash, since as you state humans generally already understand separating their social circles. It's the application of the model that people will find confusing, and will require a learning curve. It might turn out that just getting through the modelling phase successfully will be enough for enough people to sign up, and the complexity of learning how to apply the model effectively can be optional and left to the user to discover.