In this case I think it's a cargo cult phenomenon. Sometime soon after World War II our culture decided that wearing a long white coat, having a Ph.D., and publishing "real data" was the touchstone of truth in every field. As if demanding "real data" from an essayist wasn't like asking your fiancée to take a battery of psychological tests before you'll marry her.
Gladwell isn't a scientist, as he has said many times. He is inspired by scientists (among other people), but what he actually writes are book-length essays. You're welcome to argue against an essay -- that's what the form is designed to provoke -- and you can do so in several ways: You can write an essay of your own, or you can do a scientific study, or you can even perform a stunt:
After we came out of the church, we stood talking for some time together of Bishop Berkeley's ingenious sophistry to prove the nonexistence of matter, and that every thing in the universe is merely ideal. I observed, that though we are satisfied his doctrine is not true, it is impossible to refute it. I never shall forget the alacrity with which Johnson answered, striking his foot with mighty force against a large stone, till he rebounded from it -- "I refute it thus."
But the modern fashion is to respond to an essay that you don't like by demanding that it dress itself up like a Nature manuscript and then criticizing the flaws in the costume. Which is silly. The more elegant thing to do is to argue with the essay. Write an essay refuting Gladwell. Quote some scientists of your own in the process if it helps. Obviously, debating a guy with the writing chops of Gladwell is a tricky task, but it's better to try (or to remain politely silent and wait for others to try) than to flail around ineffectually.
Or, if you prefer scientific debate to essay-writing, I'm told that Gladwell seems to be trying to provide more footnotes to the science that inspires him. Follow one of those footnotes to the scene of battle, grab a handful of related journal articles, and join the fray. Don't be surprised to find every other scientist on earth alongside you, though. Scientists have a tendency to swarm fields which have been popularized.
