With Facebook, I felt as if I was on this huge football field with all of my 'friends.' I could lean in to whisper with a friend here and there, or even put on some face paint and huddle together with like-faced friends to form a group. But everyone could still see me, and I could see them – I just had to peer down the field. I can't really say things to my group that I'd normally say in private because with all these people on the field with me, someone would certainly overhear us!
With Google+ the structure is different. Rather than a field, it's more like a big building with many rooms. Each room can be decorated and tailored to a specific group of friends who hang out there. Best part is once I'm in the room, I can close the door and be myself! I can go up two levels, change hats, and walk into a different room.
tl;dr Google+ lets me fully engage my various social sides, whereas Facebook never really let you as you were always in the eye of the public.
Edit: One thing I did notice that I wish they would change is that it seems as if a friend can only be placed into one circle. Often times there's a lot of overlap among my social circles and friends may be part of at least two different groups.