One of the interesting things that has happened with the increase in inequality is the ability for more people to say, "No, I'm not rich. that guy is rich." Because as inequality rises, the people you compare yourselves to are much further away than they used to be. That's how people making 10x the median income can, with a straight face, declare themselves to be "middle class". Inequality itself is used to justify other inequality.
I agree with you that what we have is luxury compared to much of the world. Like you, I've seen this first-hand.
What I think is often missing from these discussions though, is that working a low paying job in New York City, for example, also affords considerable luxury compared to much of the developing world. That hypothetical person working that low paying job might not feel that s/he is living in luxury -- without some agreement on what constitutes basic comfort, it's quite difficult to define luxury.
The problem, which I think you've identified well, is that even the idea of basic comfort shifts as the economic landscape shifts. When the idea of basic needs shifts, the idea of wealth shifts with it.
In general, I think it's good for those of us living in privileged areas to recognize our blessings. I also think comparisons made to the developing world tend to oversimplify the problems of inequality present here as well.
Also, not to single this comment out on the middle class thing, but I've seen a few comments here which are equating middle class with median income. Middle class is a social construct. It's not well defined.
Having lived and traveled in developing nations, I am perfectly happy to call what I have "luxury". Which is exactly what most of the world would call it: http://gumption.org/1993/memo/landmarks/global_income.html