What I mean by geographic diversity is that your circles aren't limited to certain geographic areas.
Let's take Facebook, for example. Most of my Facebook friends are clustered around my immediate area, San Antonio, and Austin. That sort of makes sense. If you're adding friends and people with whom you may have some mutual acquaintances; then it stands to reason that your social graph will depend largely on your physical presence.
In this way, Facebook has somewhat of a small town feel, where everybody knows each other or is related somehow. And despite the occasional Internet famous person I follow who live outside my geographic area, everybody kind knows each other in that sphere too.
Google+, on the other hand...
In contrast, Google+ for me has been very different in the sense that I find interesting people from all over the world.
To be honest, I did "import" a bunch of my friends on Google Buzz over to Google+ when it launched. My Buzz friends were already geographically spread out through Europe, Asia, and other places. But, Google+ only seems to have continued the trend towards geographic diversity.
To be clear, I did look for local people to add into local circles. Some came from using the Nearby feature on my mobile device; some came from searching for local city names in profiles. However, the vast majority of people who have circled me are from all over the planet.
I think this geographic diversity that Google Plus offers is very similar to what Twitter offers. On Twitter, I follow people based on interest more than how I know them.
The main distinction
Google Plus is something like an RSS feed with interaction. It's like going out into the big city and meeting all kinds of interesting people. It's more of a window to the world than a window to my world. Whereas Twitter has done a great job of connecting people who would otherwise have little in common, using only 140 characters, Google Plus builds on that with additional means of expressing yourself.
You can write longer posts, add photos, photo albums, video, links, location. You can also segment your view of the world into different circles based on your interests or agenda. And, based on the recent updates by Google Plus, it seems that you will soon be able to turn passive content consumption into actual working groups of people collaborating across geographic lines. It's a synergistic dream.
With Google Plus, it's not a question of who you know; it's a matter of who you want to know, regardless of where they live.