"Oh beware my lord of jealousy, it is the green eyed monster that doth mock the meat it feeds on."
This underlies the premise for the play -- jealousy. This is seen in Iago and Othello and Roderigo, and it is what causes Iago's plot to be mostly successful.
In act. V, scene I Iago says "If cassio do remain/He hath a daily beauty in his life/that makes me ugly..." In this quote he shows the banality of his drive to destroy others. Iago shows himself to be the small person that he really is.
In Act I, scene I, Iago says to Roderigo, "I am not what I am." This is important because it openly states how Iago will behave throughout the entire play. His words and actions will all be lies. No one should trust him. Othello's fate (and through it, Desdemona's) is spelled out right there.