Deep psychological analysis of any and every aspect of themselves AND (the real jackpot) their relationships, including their sex lives.
I'm convinced that this is pretty much what drives the majority of female conversations in the world... seems pretty common in my world, at least. I have some girlfriends with whom I find it very easy to talk for several hours a day, every day. That has gone back to high school, and hasn't changed since. But I think women do have it a bit easier in the conversational-lube aspect of socialization.