Many have argued that Othello's tragic flaw is his jealousy, but that's really only the tip of the tragic iceberg (to continue the analogy). Othello, for being such a hardened and vicious warrior on the battlefield, is lost when it comes to relationships and people. He is, in a word, very gullible. He believes whatever anyone tells him and takes what they say at face value. We see this when Desdemona's father tells him that she might deceive him, as well as when Iago begins to work his dark work on the Moor. He trusts the wrong people and ultimately kills off all of those who would have been his biggest champions.
Athello's tragic flaw is that he wanted to be something he was not. He wanted to be white.