Location sharing in realtime is one feature my wife and I still use to keep track of each other. Even if I can't answer the phone (in a meeting, didn't hear it go off) she can take a look and see where I am.

Hangouts is great. Remember that Hangouts launched as a part of Google+ before breaking out.

Ironically, Google+ Photos is also one of my favorites. If you snapped photos on a vacation, you may return to find that Google has stitched them together into a "story" -- including where you were when you took photos, who you were with, etc. It's a nice way to relive the memory.

Circles is also a very simple concept for social network security. It's easy to understand, and I like that Google left it out in the open as something you should definitely do. Facebook has a similar feature, but it's not in their best interest to highlight it.