Ummmm..... Only Norway and Luxemburg have a smaller lower class than the US. Considering our population and diversity we are doing very well.
Depends on your definition of "very well".
the table I posted includes the impact of tax transfers on incomes (i.e. cash assistance, earned income tax credits, etc.) but I do not believe it includes the impact of other government programs such as healthcare and free higher education. The lower class may be bigger in the Eurozone, but those people get a lot more help from their governments than poor people here get.
That fact partially explains other comparative elements to highlight between the US and other rich countries, such economic mobility and overall happiness.
In the US, if you are born poor, you're very likely to remain poor, especially in the South.
http://www.pewtrusts.org/en/research-and-analysis/reports/0001/01/01/economic-mobility-of-the-states
And, the US has been falling for years in the "happiness index" which sounds corny but is a measure of overall life satisfaction based on sound data.
http://news.gallup.com/opinion/gallup/206468/happiest-unhappiest-countries-world.aspx
And as exhaustively discussed in other threads, Americans have worse health outcomes than other countries, despite paying many times more for healthcare. Of course, the poor in America have the worst health outcomes of all.
the point of all this, when it comes to tax policy, is to simply ask the question "what are the needs of the country, and how should we arrange our tax policy to meet those needs"? The upper income group in America is large, it's doing extremely well, it's sitting on huge piles of cash and investments, and there is no evidence at all for the idea that another big tax cut for that group is going to help any of the serious problems facing America today. The lower income group and middle income groups are heavily burdened by healthcare costs/access and higher education costs. Raising taxes or cutting their benefits to pay for a big tax cut for the wealthy is certainly not going to help.