Ok, not exactly circles like G+, but I would like to be able to create a list of users who I think give especially good answers or ask interesting questions.
So the idea is that I can go to this user's profile and have a button to add him to my list of users to follow. (And later remove if I want)
In my own profile I have a new tab where I can see all questions and answers by users who are in my list.
This would not only be a interesting new feature, but would as a nice side effect enhance the community feeling.