Ignoring all the "OMG, you guys are all communists" bullshit, I think there is a fairly easy to draw line...and it doesn't even require the assumption that acquisition of wealth is "bad".
Obviously things will never be equal, but past a certain point (that changes depending on many factors), the concentration of wealth means the AVERAGE person is getting poorer. The size of a consumer economy isn't a fixed number, of course, but the PERCENTAGE that any individual has of the economy can be determined. And if you have fewer people with a bigger piece of the pie, it logically follows that you have more people with smaller pieces. The end result is that no matter how much the economy grows, the folks who have less of the wealth than they used to are poorer even if they make more money.
This sounds a little like inflation, but it's not exactly the same thing...although the idea works pretty much the same way. Even if you adjust for inflation, a middle class salary in the 1970's was much closer to the higher end income than it is today...which means that the middle class in 1970 was (relatively) more wealthy than it is today. What's interesting is that this doesn't affect everyday things so much as it affects the kinds of things shared more between economic classes. College, for instance, is MUCH more expensive than it used to be. And as society changes, the RANGE of things we buy has increased, even if the overall price hasn't.
30 years ago, a man working any decent job, even if it was blue collar, could raise a family in most parts of the country on his income alone. Today, it's not uncommon for both parents to work to get enough money to raise a family, and they still have trouble making ends meet, even if they both work at good professional jobs. THAT'S the problem with income inequality, and why it's actually quite anti-capitalist. It discourages labor by decreasing the value derived from the majority of jobs. It's reasonable to have the dream of the big house with the big yard, but by concentrating wealth at the top end of the scale, that dream for many people is being replaced by the dream of being Bill Gates for a much smaller number of people. Motivation to be rich only works as long as it's a reasonable goal, and if the number of people in that bracket is shrinking, it doesn't matter if they are MORE rich, because most people can't reach that point.
And whining about "socialism" aside, even folks like Alan Greenspan admit this is a problem.
