The Othello which is presented to us in Act I scene i of William Shakespeare's Othello is not an exemplary character. In fact, he is painted as an ineffective leader and a disreputable scoundrel in the conversation between Iago and Roderigo, though of course he is never mentioned by name.
First, Iago makes it clear that he thinks Othello is a terrible military leader, and it is equally clear that Iago hates the Moor. (Of course, this should clue the audience in to the fact that we should not necessarily believe everything Iago says about Othello, but of course we cannot be sure yet.) He tells his friend that three prominent Venetians asked Othello, on Iago's behalf, to make Iago his lieutenant, but Othello said:
...For “Certes,” says he,
“I have already chose my officer.”
Obviously this is another reason for Iago to berate Othello, claiming that the man Othello chose for a lieutenant, Cassio, is unworthy of the position because he has no practical experience. Iago claims that the man is weak and unqualified in every way, and Othello actually saw Iago in action, so to speak, and therefore should have chosen him as his lieutenant. Instead, Iago believes that Othello played favorites.
Preferment goes by letter and affection,
And not by old gradation, where each second
Stood heir to th' first. Now sir, be judge yourself,
Whether I in any just term am affined
To love the Moor.
Here Iago has accused Othello of choosing his leaders by feelings rather than wisdom, and he has also, in his disappointment and bitterness, given himself permission to be disloyal to his leader.
Iago goes on to paint Othello as a fool because he will never suspect that Iago is only appearing to be loyal to him while always looking out for only his own self-interest.
Even worse, Iago and Roderigo go to the home of a respected Venetian senator, Brabantio, and crudely accuse Othello (a black Moor) of being sexual with Brabantio's pure (and white) daughter. Of course, Barabantio dismisses Iago at first, since Barabantio assumes Iago is just trying to get back at him for dismissing Iago's suit for that same daughter's hand. Iago calls Othello a devil and warns Brabantio that Othello will be responsible for giving Brabantio black grandchildren, thus ruining his family. It is Iago's way of taking his revenge on two people whom he considers to have wronged him: Othello and Brabantio.
Then, in scene ii, we meet Othello. We learn that he comes from a noble family and that he loves his wife, Desdemona, enough to marry her instead of just having an affair with her. He speaks calmly and reasonably, and he is quick to give grace when he can--the exact opposite of the hot-headed and impulsive Iago. When he learns that he is being sought after by the city officials, he refuses to run.
Not I, I must be found.
My parts, my title, and my perfect soul
Shall manifest me rightly.
In iii, Brabantio angrily accuses Othello of kidnapping his daughter, Desdemona; Othello remains calm, showing respect for the older man. He tells a moving story of how he fell in love with her and did not want to take her from her father but had no real choice if they wanted to be together. Even the Duke is moved by his sincerity. Othello agrees to defend Venice against some marauders, and Desdemona passionately protests her love for Othello and begs to be allowed to go with him.
It is clear that Iago is a bitter man who is jealous of the noble and gracious Othello, a man too wise to choose Iago as his lieutenant.