I think one of the main problems is that as the gap between rich and poor grows, the (for want of a better word) average gets skewed. It's human nature to want to appear successful and yet what you need to appear successful becomes more and more out of reach. In what may be an appeal to a subject close to your own heart, the more rich people you see riding around in shiny new BMWs, the more poor people you'll see buy knackered old four-pot BMWs that they can't afford to run.
It has been suggested that the effects of this are extremely damaging to society because it loads stress onto people at the bottom as they aren't able to keep up with the Joneses. This manifests itself in a few different ways - including more people resorting to crime and more people falling victim to mental illness.
Before this argument gets dismissed in the usual way, I don't want perfect equality - I'm not a communist. However I do acknowledge that the gap between rich and poor is a problem and needs to be narrowed from where we at the moment, which is an increasingly dysfunctional capitalist society.