Inequality itself is bad for many reasons. It creates a rift between people, causing feelings of envy and unfairness, increasing crime. It concentrates powers into fewer hands, and concentration of power rarely results in something good for the general population.
I can tell this article is preaching to the choir because it doesn't bother to explain why inequality is bad. It just assumes that we agree it is. So why is it bad? Especially since most Americans don't even know about it, how can it be hurting them? Two possibilities I can think of:
A) Jealousy
B) Risk of revolution
Inequality is a problem if inequality is restraining the total growth of society or limiting resources/services/goods that would potentially be available to you in a less unequal society.
An interesting TED talk on the more general effects of equality that talks about many of the issues you raise. It is titled "How economic inequality harms societies" [1] (link below). Looks at correlations between income inequality (defined as the average income of the top 20% / average income of the bottom 20%) versus measures such as homicide rate, mental illness, social mobility, high school dropout rates, prisoner counts, infant death, etc. All the data is both compared on a country level and on a US state level.
Inequality is not a bad thing. It's actually necessary to have inequality of outcomes otherwise you have no incentives for improvement. An example of a society with very low inequality is old-school communism where most people are equally poor - it's clearly not a good thing.
What is bad is extreme inequality where a lot of people are in poverty. I think most people would agree that poverty is a bad thing that we should try to reduce.
It's quite possible for poverty to decrease and inequality to increase at the same time. The old "a rising tide lifts all boats" analogy. It just means everyone is doing better but the wealthy are seeing more relative gains. If the whole pie is growing, it's easy to see how everyone could get more pie even if it's not distributed equally.
edit If you disagree please state why. Drive by downvotes do not convince me that I'm mistaken, they just lower my opinion of the average HN user.
The problem with inequality is the polarization, the haves and the have nots with no in between. That leads to a more brutal society where there is no ground for agreement.
Right, inequality isn't inherently a bad thing, its when it get so extreme that it impedes class mobility and shrinks the middle class. We have to strike a really careful of regulations and freedom to create a desirable society.
You'd think, but as it turns out, inequality has strong negative effects to the point that even the rich in an inequal country do worse than the more poor in a more equal country.
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.
Inequality in all its forms is not bad per se. Economic inequality is bad because (1) on a moral level, it is grotesque that some people have so much while others struggle immensely (and we can do something about it - money can be transferred, looks cannot), and (2) money amplifies the political voice of those who have it, undermining the concept of democracy - especially when they use that voice to direct benefits of the political system to themselves, at the further expense of the have-nots. Socialism for the rich and capitalism for everyone else, as the saying goes.
Plus, the sheer magnitude of inequality is immense in the area of wealth. It follows an exponential distribution; those other things you mentioned follow a normal distribution.
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