It's the best RPG community on the internet. There's also a lot of political discussion, science/tech discussion, and it seems to be popular with photographers and various game developers.

Google+ is being used by a lot of really interesting people with cool interests. The quality of content is very high. Well, it was, until Youtube got hooked into it.

And all that in a very freeform manner, in a way that makes it easy to find people with similar interests. To me, it's the closest thing to usenet since usenet.