I agree that Shakespeare establishes Othello's power and military prowess for the reasons mentioned in the previous posts. As epollock notes, Aristotle dictated that a tragic hero must be one who is "better" than the average person, since this sort of character trait will evoke catharsis in readers (who will pity Othello and also be afraid that if someone as noble as he is able to have such a tragic fall, then anyone is susceptible to such tragedy). The respect that all the characters (even Brabantio, before Othello married Desdemona) have for Othello make it even more shocking that he is capable of the behavior he exhibits in the second half of the play. Further, as amy-lepore notes, the race issue cannot be overlooked here; Othello, DESPITE being an outsider in Venice, was able to achieve such a high-ranking position. However, as Iago so clearly brings to light, other characters, when pressed, cannot overlook Othello's race.
It may very well have been to bring to light the racism of the time. Shakespeare has been known (as are other authors--Chaucer, Alexander Pope, Jonathan Swift, Charles Dickens, Oscar Wilde, Edith Wharton, to name a few) to put it all out there and place the mirror before society's face and make them look full-on into their own faults. He often addressed the way women were treated in his time period--why do you think so many of them dressed up like men and went out into the world to do the things they did? Because they couldn't do them as women! Sometimes it works, sometimes it doesn't.
As a tragedy, Othello had to be in a heightened position at the beginning of the play. Although to be too narrowly concerned with the Aristotelian definition will limit the play's emotional effects.
Shakespeare's use of a general revealed many themes which are almost too numerous to mention here. But the fact that he made him a Moor also highlighted many other subjects such as race, and being an outsider, that Shakespeare felt were important for audiences to see. The play has so much to offer and there is something in it for everyone. Othello is one of his most critically acclaimed plays, and is a staple of most AP and IB classrooms.