Stephen Dubner comments on complementaries. This was written on a sign he recently saw at an Italian restaurant in London:

Heaven is where the police are British, the cooks are Italian, the lovers are French, the mechanics German, and it's all organized by the Swiss

Hell is where the cooks are British, the mechanics are French, the lovers Swiss, the police are German, and it's all organized by the Italians!

A Euro-centric view of the world - but still a great point.  What's interesting to me is that despite the obvious positives of diversity, many of us seem to feel more comfortable with people who look and act similar to the way we do. Even though it's more beneficial to live and learn from those different from ourselves - our inherent sense of wanting to belong and "fitting in" often dominates.  This is certainly not true in any absolute sense, but on average it's probably true for most people.