In Piketty's credit, he did make this easy. Providing data for reproducibility and replicability is relatively new in economics. There are, however, two kinds of claims here. One is that mistakes were made, which is grounds for schadenfreude for those (like me!) who don't like the slight jumps in reasoning from "inequality is rising" to "something must be done about inequality" to "this is what must be done". For all…
The total effect of his cherry picking can't even be discerned from this article. With a lot more work (more than a single blogger probably has time for), it might be shown to be much worse. For example, Piketty chose the U.K., France and Sweden for his list of European countries (averaging them all into "Europe" without adjusting for their populations, as the article pointed out). But why those particular countries? Would the results have been completely different if he would have also included Germany, one of the wealthiest and most populous European countries? We'll probably never know how he came up with his set of countries, and whether he deliberately omitted other countries whose data was available because they would have caused his results to be not significant enough to publish (or even contradicted his thesis).
This reminded me a lot of one of the original studies linking cholesterol to heart disease, which chose a carefully picked subset of European countries even though results were available for several others. Subsequent research showed that including the data from the missing countries would have yielded very different results.
