Malcolm Gladwell is great at writing anecdotes, but he dangerously masquerades these as science. Case studies can be incredibly informative -- they form the historical foundation of cognitive neuroscience and continue to be an important part of cutting-edge research. But there is an important distinction between science, which relies on structured data collection and analysis, and anecdotes, which rely on an entertaining narrative structure. His claim that dyslexia might be a "desireable difficulty" is maybe the most egregious example of this. Mark Seidenberg, who is a leading scientist studying dyslexia and an active advocate, has written an excellent commentary about Gladwell's misrepresentation of dyslexia. The short version is that dyslexia is a serious problem that, for the vast vast majority of people, leads to various negative outcomes. The existence of a few super-successful self-identified dyslexics may be encouraging, maybe even inspirational, but it absolutely cannot be taken to mean that dyslexia might be good for you.
In various responses to his critics, Gladwell has basically said that people who know enough about the topic to recognize that (some of) his conclusions are wrong, shouldn't be reading his books ("If my books appear to a reader to be oversimplified, then you shouldn't read them: you're not the audience!"). This is extremely dangerous: readers who don't know about dyslexia, about its prevalence or about its outcomes, would be led to the false conclusion that dyslexia is good for you. The problem is not that his books are oversimplified; the problem is that his conclusions are (sometimes) wrong because they are based on a few convenient anecdotes that do not represent the general pattern.
Another line of defense is that Gladwell's books are only meant to raise interesting ideas and stimulate new ways of thinking in a wide audience, not to be a scholarly summary of the research. Writing about science in a broadly accessible way is a perfectly good goal -- my own interest in cognitive neuroscience was partly inspired by the popular science writing of people like Oliver Sacks and V.S. Ramachandran. The problem is when the author rejects scientific accuracy in favor of just talking about "interesting ideas". Neal Stephenson once said that what makes a book "science fiction" is that it is fundamentally about ideas. It is great to propose new ideas and explore what they might mean. But if we follow that logic, then Malcolm Gladwell is not a science writer, he is a science fiction writer.