The difference between Gladwell and Diamond (and for that matter Levitt) is that Gladwell is attacked for his work being atheoretical; it's basically a collection of anecdotes that belongs on a bookshelf with Bill Bryson.
Diamond is criticised for stretching his grand unified theories too far and missing all the evidence to the contrary. Levitt is accused of picking bad hypotheses and doing sloppy econometric analysis to support them. Gladwell doesn't have any grand unified theories, or any theories substantial or original enough to be controversial. and doesn't do any comparable statistical analysis.
Diamond is criticised for stretching his grand unified theories too far and missing all the evidence to the contrary. Levitt is accused of picking bad hypotheses and doing sloppy econometric analysis to support them. Gladwell doesn't have any grand unified theories, or any theories substantial or original enough to be controversial. and doesn't do any comparable statistical analysis.