A Google+/Facebook observation: Having to select circles to "share" posts with somehow makes it feel like spam, like I'm targeting people. And then I imagine their individual reactions; I imagine a lot of cringing at whatever inane thing I've said. On Facebook, the wall metaphor suggests that people voluntarily come to check out what you've posted. But here, a post targets specific people and is on a "stream"—a moving channel from me to them—making it feel like a more active and intrusive form of communication.
Ironically, I'm sending this to all of my circles.