There is an interesting book, "The Spirit Level: Why Equality is Better for Everyone" by Richard Wilkinson and Kate Pickett, that makes a very strong argument that in developed countries inequality is correlated pretty much every "bad" metric (health, crime, teenage pregnancy, etc.) of a society.

Now, of course, correlation does not mean causation and it's perhaps more interesting what the true underlying causes are. But if inequality is such a strong indicator, it is worth wondering whether inequality isn't a bad thing in itself.