Hamlet's fatal flaw is often said to be procrastination, which is another way of saying indecision. He hesitates and questions the rightness of his actions, and also seems unsure if his motivations are pure. Part of the reason why Hamlet is unable to commit to course of action lies in his relationships. He is unreasonably close to his mother, causing him jealousy and torment; some productions (such as the one in London starring Daniel Day Lewis as Hamlet and Judi Dench as Gertrude) have even suggested an incestuous and physical sexual relationship between them. Certainly there are suggestions to support such an interpretation in the text. The amplified emotions he feels about this relationship definitely affect his ability to act decisively, and this is also reflected in his behavior towards his lover Ophelia.
Hamlet's tragic flaw (the Greeks called it "hamartia") is SUPPOSEDLY his inability to take decisions. This has been forwared by many critics, including the famous A.C.Bradley in his classic book on Shakespeare, "Shakesperean Tragedy: Lectures on Hamlet, Othello, King Lear, Macbeth" (1905). Since then, Shakesperean critics have taken this view more or less for granted.
I think the question we should ask these days is WHAT made Hamlet so indecisive. Here, I find the previous responder, Appletree, helpful in suggesting Hamlet's various complexities, especially regarding women, Gertrude (hi mother), and Ophelia (his lover). Perhaps for reasons of space, Appletree does not go deep enough to discuss Hamlet's misogyny (hating women).
My daughter wrote her senior theses last year on Hamlet. There she claims that Hamlet's indecisions result from his intellectual capacity to doubt -- almost everything. Now, this is also not a new idea. What was interesting about her thesis though is the fact that she cast doubt as an intellectual exercise, something Hamlet was fond of indulging in as a student, and then becoming obsessed with the act itself.
In the Elizabethan age, the age in which Shakespeare wrote, intellectual doubting was becoming a fashion. Hamlet was most probably produced between 1599-1601. At that time Shakespeare was about 35 years old, and, the voracious reader that he was, it is a safe bet to say that he was acquainted with the works of Michel Montaigne. He was a French aristocrat and intellectual with wide influence all over Europe. He published his book of "Essais" (Essays) in 1580. It was widely read among intellectuals in England.
In that book, Montaigne teaches intellectuals that doubt, unlike popular religious prejudice against it, was intellectually necessary. To Montaigne, doubt was a natural reaction for anyone who plainly sees hypocrisy.
Remember Hamlet exclaiming to Gertrude, "Seems mother! Nay it is, I know not seems!" early in the play? Rather than sexual jealousy -- which may have been real in Hamlet's case -- Hamlet's doubts came about by seeing a constant gap between what people say, and how they behave: Claudius, Polonius, Gertrude, even Laertes. Hamlet's episodes with the ghost of his father also creates doubts -- not because it was a ghost, but because as a father, he would try to oblige his son to murder his uncle.
So, if you were to write on Hamlet's indecision, please pay some attention to his doubts. It may raise your essay from ordinary to interesting.