A tragic hero typically is a human with an excess of a particular virtue or other quality. Often, the quality is hubris, or pride. One can have too much of a bad thing, or, conversely, too much of a good thing. The problem for the tragic hero is that this hamartia is like a fatal flaw--it causes the hero to make choices that bring about his downfall and the downfall of everyone else around them.
The beauty of Othello is that so many of the characters are tragically flawed. Othello is the most obvious. He is blinded by his love and his trust in the honesty of his men. He is a general, a man who is astute in battle, and who is not likely to be outwitted on a battle field, but he knows little about the workings of people. It is ironic that a man who is most frequently misjudged on the basis of his moorish appearance is also tricked into judging Desdemona on superficial appearances.
Another possible tragic hero is Iago. He is smarter than most, and more cunning. He has a way with people, and is able to trick most people into thinking he is honest. His skill is in knowing people's weaknesses and using that weakness against them--with Othello, he uses his absolute faith in Desdemona, with Cassio his weakness for wine, with Roderigo his infatuation. He even uses Desdemona's kind hearted nature to work his evil. His flaw, his weakness is his insecurity. He is jealous of everyone and considers people guilty without asking for proof.