What are the main themes in the play Othello?

I would have to go with the theme of duality as one of
Shakespeare's most important ideas in the tragedies: what appears to be reality is often
not. Notice how often the word "seem" is used in the play: "Men should be what they
seem." This familiar lament is ironically stated and discussed repeatedly by Othello and
with, ironically, the most two-faced character in the play. It happens over and over in
the plays. Here, it ends in total disaster for Othello, who is far too trusting to
understand the reality of Iago. Othello's simplistic military code is no match for
Iago's Machiavellian manipulations.

The second most
important theme would then be the nature of jealousy, one of our most primitive
emotions. Shakespeare explores its permutations with 3 different couples, all of whom
have one partner who is fairly consumed by it and the other who deals with it as it
affects their relationship. Ultimately, it destroys or at least seriously hurts all 3
couples. Desdemona and Emilia are basically killed because of it. Shakespeare has many
penetrating observations about this emotion. The one that strikes me is that he says
that jealous souls are never jealous for a reason, but because they are jealous. In
other words, there is no good reason for them to be jealous. They just are. That is
scary stuff and well worth considering as a warning.