Its a scary thought to have one company control almost all the data for a user. Just think what kind of data Google is collecting using their search engine, adsense, gmail, chrome etc. And now if they have users also move their social circles to Google+, it will give them almost complete information about a person. This is for them THE missing piece of the puzzle to tie everything together. Just for this single reason, I am never going to join Google+ however fancy and cool those circles look like.