This NME article, entitled "Why Malcolm Gladwell Is Wrong About The Beatles," offers a brief rebuttal to the author's theory from Outliers that "'genius' has more to do with hard work than any innate genetic destiny." Vis a vis The Beatles, Gladwell's thinking goes that the band's time in Hamburg, where they really sharpened their skills as musicians (but not necessarily songwriters), helps a lot in explaining their later creative triumphs.
In this debate, my own sympathies would lie with the contra-Gladwell side. I think it's folly to try to rationalize The Beatles' greatness largely as a component of sheer numbers (i.e. the "10,000-Hour Rule"). Hard work is certainly critical to the gestation of "genius," but for Gladwell's purposes, it's not the place to start and it's not the place on which to put this degree of emphasis. In reality, the task that Gladwell laid out for himself was almost inevitably going to yield inadequate findings. You can't satisfactorily nail down genius by gathering statistics, dates, etc. You can't satisfactorily nail it down at all. You can construct a learned context but the full picture will never emerge. Creative genius is elusive. Doubtless, Gladwell knows this. But short of attempting a bold insight (which in this case would almost certainly stem from grasping speculation and shoddy theorizing), his views would seem bound for the clinical and obvious.
The Beatles' success resulted from a confluence of factors: their coming of age as the rock 'n roll genre took shape, the concomitant rise of music as a commercial enterprise, the flowering of studio technology, the band's individual skills and how those skills meshed with each other's, their work ethic, etc. But, of course, they also possessed something or somethings that defy calculation and easy classification. A certain je ne sais quoi that you know exists simply because you know The Beatles. It's the reason why The Beatles are The Beatles and not the Stones or Zeppelin or the Who. And it's the reason why Gladwell's theory will, in all likelihood, always come up woefully short.
(P.S. I have not read Outliers and don't intend to. Based on this admission, feel free to discount the above arguments).