There are two factors that you can interpret out of that graph, or other income distribution graphs, one is that a lot of people own very little of the money and a few people own quite a lot of the money. This is a qualitative conclusion, and questions addressing it should take the form of questions about inequality.
There's also a quantitative question, what is wrong with this magnitude of inequality specifically, and is there a minimal magnitude of inequality (or a similar quantity) which makes the problems of inequality go away? Alternatively, do injustices and unfairness scale with the magnitude of the inequality; and if so, how?
The first one is a lot easier to address. It's quite well understood that inequality within a community correlates
very strongly with all crime rates (except for arson, weirdly). Poverty and inequality have a
mediated relationship, but in the US they correlate quite strongly over communities. There's also strong correlations with poverty/inequality and ethnic diversity and proportion of non-whites in a community. I studied the data for this a few years ago.
I aggregated census data in the US with crime data and income data, extracted communities which are measured in each data set, then dida
principal component analysis of crime, make a coordinate system with the first 3 principal components (of which the first and the second are the most interesting). Plotting demographic variables and measures of inequality/poverty over the sphere gives you a
colinear relationship between poverty, inequality, ethnic diversity and non-white population proportion. Which is to say that in terms of variation in crime rates, (inequality)-(poverty)-(ethnic diversity)-(non-white proportion) are a single construct. Or more prosaically 'race and class are two sides of the same coin'.*
This isn't to say that such an analysis lets you draw causal conclusions from the demographics, in fact it impedes treating inequality, poverty, ethnic diversity and non-white proportion as separate quantities for causal analysis. Which isn't to say it's not possible, it's to say that their alignment gives strong evidence of pairwise
mediation, or in another word, confounding, when trying to draw causal conclusions.
Some analysts
also throw economic growth into the mix as an inflation factor of inequality. Prosaically, money makes more money. It's interesting to ask where the money's made - what kind of human activity generates the biggest changes in ownership of monetary value?
This is hands down the stock exchange or financial sector. Worldwide, in total about 60 trillion Euros are traded in stock exchanges per year. This is more than the combined monetary value of all goods and services bought in a year worldwide. See
here for World Bank data on the subject. Wealthy countries have a bad habit of trading more monetary value in stocks and financial assets than their entire yearly GDP.
It's interesting at this point to note that the idea 'those who demonstrate consistent skill and utility to their communities are reward financially', that
@Agustino said, is pretty much bunk considered under this light. The majority of US stock traders
are worse than chance at their job, but
are disproportionately rewarded to the tune of a wage equal to about twice the median income in the US.
Who actually has the money to invest in stocks, to play the game that generates most wealth? You need
about 3000 dollars to purchase an index fund.
This breakdown of the 'average cost of living per year', they have enough to minimally purchase index funds or speculate on the stock market. But what about people receiving the minimum wage of 7.25 dollars per hour? They can't even afford the average yearly expenditure on food+housing in that breakdown (approx 16k dollars per year before tax). So they're locked out of the mechanisms by which the economy grows, so will receive less and less of a share. Conversely, those who get to play the game will get more and more of the share. The poor also can't pay for lobbying, so their political impact is diminished as well.
So what's so bad about income inequality in general? It correlates strongly with racial inequality, inequality of opportunity, it locks out the poorest from growth, it promotes crime.
What about the quantitative question at the start of the post? How do greater magnitudes of inequality impact a nation? Well.
inequality has a strong link with crime rates, more inequality more crime. Also, growth magnifies unequal distributions of monetary value to the extent that the the monetary value is unequally distributed. So if we have a
monotonic relationship between X and a set P(X), increasing X increases P(X) - so growth is likely to increase the inequality in income, which through its correlates is likely to increase poverty, racial inequality, crime and decrease social mobility and political agency of the poor. Since these relationships are linear (see collinearity earlier, linear is a subspecies of monotonic), the more unequal things get, the worse the expected impact on poverty, inequality, crime, social mobility and political agency.
*the angle between the first principal component and all those vectors was between 5 and 15 degrees, perfect correlation is 0 degrees, no correlation is 90. The vectors were pointing along the first principal component, this means that they all contribute/explain the major components of variation in crime over the US.