Update (11:53 a.m.): To be perfectly clear, what I am saying is not that Thomas Piketty is wrong to focus on inequality (please be careful when I use poetry - I don't intend for it to be read literally). Piketty is good in his descriptive analysis from what I have read so far. Also, from what I understand, his empirical work is very sound and quite good. It is not his descriptive analysis that I fault but rather his prescriptive analysis.
First, the world tax concept proposed by Piketty is not a workable one. I don't see how the mandate for something like that could be obtained. Second, we must keep inequality in perspective. The fact is that the poor in America live much better lives than even the upper class did 200 years ago. They typically own one or two cars, have at least one TV, possess at least one cellphone, and live in a reasonably sized home. In fact, the house of the average poor person in America is larger than the house of the average European. This is not to say that being poor in America is easy. It is just to say that we must keep things in perspective given the economic history of the United States over the past 200 years. Third, taxes are opposed by a significant cross section of American society - even taxes for other Americans. This is a matter of conflicting economic viewpoints or philosophies. If your viewpoint is that the government's role is to provide equal opportunity for all, then you would come to one conclusion regarding the proposals in the book. If you think - as Obama and most liberals do - that the government is within its rights to tax more (or, as some would put it, to engage in 'forced redistribution of wealth' or, again to put it less gently, to seize even more wealth than it already does), then you would come to a very different conclusion regarding the prescriptive proposals in the book. Given what we know about the distribution of political opinion within America, it is highly unlikely that many will take kindly to the concept of a world tax - if at all, they would be for more tax revenues to help other Americans. The fact that so many are opposed to taxes (one of the many of the ideas expounded in the book) simply indicates their political viewpoints on forced wealth redistribution. No matter how you phrase it, everybody knows that means. Nobody is getting fooled.