Iago's first complaint about (General) Othello is that Othello knew Iago wanted to be his lieutenant. Iago had even gone to the trouble to get "three great ones of the city" to speak with Othello about it. The problem is that Iago had already chosen his lieutenant--meaning Iago was passed over for the promotion--and to rub salt into the wound, Othello had chosen Cassio (a mathematician, "That never set a squadron in the field, / Nor the division of a battle knows / More than a spinster"--not a tried and proven soldier like Iago). Worse, Iago feels he has been insulted by being appointed Othello's "ancient," an archaic word for "ensign," meaning "flag-bearer." Being appointed ensign isn't really an insult; it's an honorable rank, but Iago feels slighted, nonetheless.
Iago essentially sighs and says it's just the way the military works--"tis the curse of service, / Preferment goes by letter and affection." That is, it isn't based on merit; it's who you know (this is still true today).
He doesn't outright say it, but his comments in this scene also suggest that he is racist. He refers to Othello as "the Moor," which isn't bad, but then "thicklips," "old black ram," and "an old Barbary horse."
Finally, he admits that "I do hate him as I do hell-pains." This may be because Othello did not offer him the promotion he is convinced he deserves more than Cassio, but Iago's comments later in the play suggest that Iago needs no reason to hate; he is evil for the sake of being evil.