I wish I could nest circles on Google+. I have a circle for relatives and a circle for family. My family circle should automatically be a part of my relatives circle. Similarly, I have a circle for my programmer friends and a circle for my tech-savvy friends. Instead of having to add all my programmers friends to both the circles, I wish I could just drag the entire programmer circle into the tech-savvy circle.
Circles can be a lot more powerful than just groups or tags if you allow circles to contain other circles. Technically, this feature is possible (though it might not be trivial to implement). In the past Gmail did not have nested labels but now it does. So I'm pretty sure that if enough people clamor for this feature, it will be added.
If you didn't get the joke in the image, see this.