So, basically, Superfreak's math problems aside, many economists believed that the compression of inequality in the post-war period was the product of increasingly developed economies and the result of natural economic processes. Basically, the idea was the inequality increases as economies advance from agrarian to industrial and then inequality decreases as the industrial economy further advances (that's what the Kuznets curve looks like). Piketty's book, looking at hard data from the past 200 years says that the post-war compression in inequality was not the result of natural economic processes, but an aberration. As a result, we shouldn't expect that inequality will naturally decrease any time soon and that it will only further increase.
The really weird thing about SF calling Piketty a moron and an idiot and all that is that Piketty and SF aren't really at all in disagreement about what is happening and will happen in the future (inequality will continue to grow). The only real disagreement I see is that Piketty thinks ever-increasing inequality is a bad thing and that governments should do something about it, while SF either doesn't think it is a problem or that government can do anything about it.