The Twitter model is that one user publicly follows another.
Google+ is a lot more complicated than any of these.
While using the snazzy animated Circle-creation tool may come more naturally to others (early adopters seem to be mad with love for Google+), I think this is likely to be a stumbling block for many people.
Perhaps digital relationships won’t be naturally nuanced and eroded over time like real-world relationships, because digital things just don’t do that. They exist, or they do not.
And it may just be that privacy is incredibly difficult to illustrate and conceptualize. But lots of things seem hard at the start; maybe we as humans will teach ourselves to understand this stuff better over time.