Having the hero of the play be a Moor created a character who was exotic and noble, yet for white western Europeans, his African origins suggested a savagery to go along with his nobility. The fact of his interracial marriage creates conflict, although probably not as much for Elizabethan audiences as for modern viewers.
Othello struggles with great internal conflict, fanned by Iago, over his suspicions that his white wife, Desdemona, might be unfaithful. This struggle parallels the conflict of a dark-skinned foreigner holding a high station in Venetian society.
Iago is virulently fixated on ruining Othello, probably because Othello passed him over for promotion. Furthermore, Iago suspects that Othello has slept with Emilia, Iago's wife. Iago refers twice to this suspicion, first in Act I: "I hate the Moor, / And it is thought abroad that 'twixt my sheets / He has done my office" (iii.388-390), and again in Act II:
I do suspect the lusty Moor
Hath leap'd into my seat; the thought whereof
Doth (like a poisonous mineral) gnaw my inwards;
And nothing can or shall content my soul
Till I am evened with him, wife for wife.
Iago projects his own inadequacies and insecurities onto Othello, underpinning pure hatred of his superior officer.