One of his biggest is the "survivor fallacy", where he talks to five successful people, identifies a common component and announces "that's the reason they made it!" The task of looking at the entire universe (successes and failures) and seeing how often that component exists in both groups is a task Gladwell leaves to muggles.
If I have to pick a book, I'll pick "The Tipping Point", because it was his first big success, and he makes more effort to ground his thesis in reality. The more acclaim he gets, the more frothy and goofy the next book becomes.
In his latest book, he explains how a basketball team of 10-12 girls from "from Menlo Park and Redwood City, the heart of Silicon Valley... the daughters of computer programmers and people with graduate degrees... [who] worked on science projects, and read books, and went on ski vacations with their parents, and dreamed about growing up to be marine biologists" were the underdogs against teams from ghetto schools, whose players grew up in poverty, dealt with the impacts of gang violence and often didn't have enough to eat.