Interesting thoughts.
I, too, have not been able to systematically connect to G+.
Not sure i can nail down the reason as clear as you state it (walled garden). I think it's more about connecting to it after I already has well established networks on linkedin, twitter and fb. One moe is just too much to handle right now.

One note though about the centrality of you blog as you describe it. Obviously it makes sense from the perspective of the P in PKM.
However, reading this post it felt to me that its about the feeling of ownership of the sense making. Doing that in communities where you have as much or as little control as other members of the discussion means giving up the control and ownership of the sense making. Indeed it may pull the discussion to places that make less sense to you, but in other cases I found it to actually broaden my capability to make sense in just my own walled garden. :-)