Mark Thoma worries about an ossified America:
A vibrant middle class is essential to mobility. For many people, the middle class serves as a steppingstone to the upper classes both within and across generations. If we continue along the path we are on to an increasingly two-tiered society, and if the middle class continues to experience problems, then there will be less room at the middle tiers of society for those who are trying to move up the ladder. Mobility between the classes will be reduced.
Paul Kelleher thinks Thoma isn't primarily concerned with inequality:
Reading his article, I don't see Thoma complaining about income inequality as such. Rather, I see him worrying about two more specific things. First, he is concerned about the power that those at the top often have to bend public policy toward their interests. Second, he is concerned about stagnant or even reversing living standards for those at the bottom and the middle. These two issues are connected to income inequality, but they are connected only contingently. For it is possible to imagine a society where the living standards of everyone improve significantly and steadily, but where those at the top improve faster than others, and so inequality grows. It is also possible (although perhaps harder) to imagine policy changes that hem in the political clout that greater wealth buys, so that political influence becomes somewhat disconnected from economic status.
(Chart via N. Asher)