battle knows
More than a spinster; unless the bookish theoric,
Wherein the toged consuls can propose
As masterly as he: mere prattle, without practise,
Is all his soldiership.

Iago believes that this unqualified candidate has been appointed to the post because he is from a higher social class than Iago. He later says that he hates Cassio not only because he occupies this post, but also because he always makes Iago feel inferior.

He hath a daily beauty in his life
That makes me ugly

Even in act 1, scene 1, Iago is made to feel inferior again, not to the accomplished Michael Cassio but to the fool, Roderigo. The two assail Senator Branbantio with assurances that Othello has taken his daughter. Brabantio refuses to listen to Iago, who speaks to him in coarse, vulgar language:

Even now, now, very now, an old black ram
Is tupping your white ewe. Arise, arise;
Awake the snorting citizens with the bell,
Or else the devil will make a grandsire of you.

However, he eventually acts on the word of Roderigo, whom he does not like and has forbidden to see Desdemona but who is at least a nobleman of approximately his own social class, not a "profane wretch" like Iago.

Even Othello's race is not a barrier to his ambitions as Iago's social class is to his. Iago betrays this every time he opens his mouth. In Othello himself, we see the intersection of race and class. Although a general, he can never be one of Venice's inner circle like Brabantio, or even the foolish Roderigo, despite his ease of address with the Duke and senators. In the first act, therefore, we see a web of class relationships between Brabantio, Desdemona, Othello, Cassio, Roderigo, and Iago already causing tensions which are to have fatal consequences for at least half of them.