The main problem is in political representation. In a democracy, everyone should count as one vote. The natural desire of the population is to have better education and health care, for example, that can lift the standing of the whole society. When a small minority has a disproportionate amount of wealth, they can relatively easily bend politics to their personal wishes. In the US this presents itself in politicians who vote for their wealthy donors to receive tax breaks, the rise of chartered schools, and the fight against public health care.