Most other features of Google+ are powerful and well planned - like video calling (hang-outs), seamless integration with other Google products (Gmail and You-tube), and a good cross-platform HTML mobile app. Still, Google is touting circles as the key Google+ feature. Why does Google think circles is so important?
Lets look at how Google+ circles affects the users social networking experience. By gently forcing the user to select which circle a new contact should belong, Google+ amortizes the job of categorizing contacts. On the other hand new contacts usually end up in one big "friends" bin in FB. The categorization (or binning) has to be performed later (and this is a tedious task - at least I haven't bothered to do it until now).
With Google+, I've ended up with my contacts being in one of these circles:
Fig. 1: Each circle is a post-box to send messages to a specific contact category.
So now I have a bunch of post-boxes, one corresponding to every circle, where I can post information (pictures/status updates/etc.) and they will get routed to that sub-set of contacts which comprise the circle. This gives me the ability to target information to relevant parts of my social network. I look at this as a social graph routing mechanism. Circles are routing rules that users put in place so that their social message streams are routed appropriately in the social graph.
Google+ is constructing humanity's social router via circles which will be programmed via routing rules defined through the elegant circles abstraction. Yes the same thing can be done with FB, but FB never really tried to make this the center-piece of its product. By gently forcing users to separate relationships via circles, Google+ might just manage to make users feel more confident about selectively routing their social lives with different groups of contacts, rather than blasting messages to everyone they (do or do not) know in their huge FB friend lists. The result should be a more information-rich Google+ social network. With more information comes better advertisement targeting possibilities.
Users are more concerned about privacy with respect to their contacts (my family should not see what happened in the office holiday party) rather than Google knowing every intimate detail of their lives. A functional social router will implement this wish without coming in the way of Google obtaining user information. No I don't think Google will be more discerning than FB when it comes to monetizing the private information of users, but hey, who cares about user privacy anyway?