I believe in one of his interviews (the New Yorker?) Gladwell kind of sheepishly admitted that people think he's a genius while really he's just packaging up other people's work with a more engaging presentation.
he's just packaging up other people's work with a more engaging presentation
Since it's pretty so rare to find someone who both (1) spends the time to find and understands that work, and (2) has the literary ability to engagingly present it, by some definitions, that can be considered genius.
Terman et. al's studies of gifted children (http://books.google.com/books?id=MXwVAAAAIAAJ&printsec=f...)
Sherar et. al's study of physical maturity in hockey player development (http://www.informaworld.com/smpp/content~content=a778078133~...)
Flynn's book "On Intelligence" and review of the growing IQ of the worldwide population and ethnographic studies of Chinese. (http://books.google.com/books?hl=en&lr=&id=qvBipuypY...)
I don't think one should claim an insufficient justification of his hypotheses without actually examining the justification thoroughly.
Oftentimes, for Gladwell in particular, people claim that he's being unrigorous, imprecise, or incomplete. He's not being incomplete. He's being concise. If he weren't concise, almost nobody would pay any attention. Almost nobody would learn anything from him. And almost nobody would, wanting to learn more, have the opportunity to follow the reference list that he carefully puts out. Which I guess few people do, anyway.