There are several main ideas in Othello. The most obvious, is, of course, racism, and the dehumanization of Othello by Iago and Desdemona's father by the repeated use of animalistic imagery is an excellent example of that (the most famous is the 'black ram tupping the white ewe" quote).
Other themes or main ideas you might consider:
Soldiers returning from war and re-entering the domestic sphere. There have been excellent arguments made for the idea that Iago caused all of the trouble because he was bored when not facing the urgency of war. Also, Othello's blind trust of his male companions and his lack of understanding or faith in Desdemona indicates that his faith still lies in a war-time military construct, in which solders trust one another with their lives.
The lies/truth dichotomy. Iago refers to himself as "Honest Iago, " and calls Othello a liar with great frequency,a nd yet Othello is one of the most honest characters Shakespeare created. He seems not to even understand the concept of dishonesty. Iago, on the other hand, never tells the truth (or not the whole truth).
Another major themeis Jealousy. Roderigo is jealous that Othello married Desdemona, Iago is jealous that Cassio was promoted and he wasn't, Bianca is jealous of the women she thinks Cassio is seeing, and Othello is jealous of idea of Cassio being with Desdemona. Without jealousy we there would be a story. Iago would have never told Barbantio about Desdemona and Othello and without that inciting moment we would not have a plot.
Ego/Pride is also a major theme. Iago feels that he deserved the promotion over Cassio and because he didn't get it his pride is wounded. Also Othello is a very proud man. I feels accepted into Venetian society because of his good reputation. When he thinks he has been cuckolded by Desdemona his pride is lost and this causes his downfall.