DiCaprio was just fine as Amsterdam Vallon, son of the murdered leader of an Irish gang, who's out for vengeance (and political clout). However: one of the virtues of the film is that it doesn't have any cheap sentimentality. A consequence is that it constricts Leo's two opportunities to raise the emotional level--his affair with Pickpocket Jennie (Cameron Diaz) and his near-adoption by the "nativist" gang leader and arch-villain Butcher Bill (Day-Lewis). It's not a shortcoming of his, it's built into Scorsese's approach to the film.