First, while the reviews, critiques, appraisals and more of the English version continue apace in both the specialised and lay media, it's interesting to observe that the original French only attracted a modest specialised readership.
The explanations put forward by these reviewers for the English version's staggering sales include the extremely timely date of publication, the clarity of Piketty's writing, the freshness of his discourse, his transparency about the assumptions and the fact that he put all of his data and spreadsheets on a public-access website so that anyone interested can check and double-check his work. All of which is perfectly true. But what a pity that so few have noticed just how much praise translator Arthur Goldhammer deserves...
Given that seamless fluency renders the translator invisible, a (monolingual) reviewer is unlikely to go beyond a passing observation about seamless fluency, if that.
An aside: Piketty's innovative methods led Cory Doctorow to post on BoingBoing a piece Thomas Piketty's Capital in the 21st Century includes the following:
... Indeed, one of the most entertaining episodes in the debate so far has been The Financial Times affair, where the FT's Chris Giles pointed out a bunch of "errors" in Piketty's work, only to have the normally even-keeled Piketty come back with a long, detailed rebuttal that boiled down to "Hey, asshole, if you'd bothered to look, you'd see that I documented every one of the decisions you're characterizing as an error, and if you want to disagree with me, then argue with my explicit, detailed assumptions instead of sloppily assuming I didn't even realize I was making them."