We see Othello not as great man but rather as a very flawed individual who happens to be an outstanding warrior. Self-delusion is at the core of his character. His greatest flaws are his refusal to face the reality of his nature and his firm belief that he has the right to be judge, jury and executioner. Watching him we learn the importance of humility, of constantly questioning our motives, of knowing and admitting our weaknesses and failures and of constantly striving to improve ourselves. We are also reminded that no one except a legally appointed person, operating within the framework of a humanistic and ethical code, has the right to pass sentence on another human being.
In regard to Iago - unlike the standard portrayal of a dispassionate, unmotivated and coldly objective creature we see an obsessive, irrational, subjective, compulsive, reckless, maniacal paranoid who is overwhelmed with hatred and yet has the incredible ability to present to the world an entirely different personality. He literally thrives on danger and excitement.