Well, I haven't been back over there since I set up my account, but I sure hope there is a way to prevent people you haven't approved or don't know from following you.
I understand that you can tailor your audience for certain posts with the circles thing...but I don't really get how your privacy will be maintained when anyone who wants can follow you...and (I assume) see all of your profile info.
Anyone can follow you, but you can hide nearly all information from people who you aren't following. There's unfortunately not a global change option so you have to set each item individually. But if you want, you can set everything to only be seen by one of your circles and then only share everything you post with just that circle. The entire world can still follow you but they won't see a single piece of information beyond your name if that's what you want. You can even set it so that people who are in your following circles don't see anything in your profile.
Like Facebook, you can preview your profile to see what it looks like for other people which helps when customizing it.