What is the tragic flaw of Hamlet?
Concerning your question about Hamlet's tragic flaw in Shakespeare's Hamlet, I'll just elaborate or add another dimension to the excellent answer above.
Teachers and critics have had fun for centuries arguing about "why" Hamlet waits, or procrastinates. Everyone pretty much agrees he waits too long, and this leads to the blood bath at the end of the play, but disagreement exists as to why he waits. For instance, in the speech quoted above, the quote given seems pretty absolute. Yet, by the end of the speech Hamlet concludes by mentioning the issue of the ghost's identity. He indicates that he is not sure if the ghost is really the ghost of his dead father, or if the ghost is a demon in disguise who intends to mislead him. (Remember that the witches do that to Macbeth in the play of the same name.) Thus, Hamlet will use the traveling "players" to determine with certainty if the king is guilty or not.
Once the king's guilt is certain--after he overreacts to the death scene that depicts a murder being committed in the same way the ghost says Hamlet's father was killed--Hamlet has a perfect opportunity to kill Claudius, but he refuses to do it. He refuses because he thinks Claudius is in the process of confessing his sin and he doesn't want to send Claudius to heaven.
So does Hamlet wait because he is unsure of the ghost's identity, or because he "plays God" and wants to send Claudius to hell, or both? Is there an underlying facet of Hamlet's personality that is responsible for the thoughts and decisions that lead to his delay? Is he depressed? Is he afraid? Does he think too much? Is he basically nonviolent, and capable of attacking only as a gut reaction, as when he kills Polonius and eventually the king?
This is just the beginning. There are more issues than I've mentioned. But the answer to why he waits may be more essential to understanding the character of Hamlet, than is just the fact that he procrastinates.
Thus conscience does make cowards of us all,
And thus the native hue of resolution
Is sicklied o'er with the pale cast of thought,
And enterprises of great pitch and moment
With this regard their currents turn awry,
And lose the name of action
The above quotation from Hamlet's famous "to be or not to be" soliloquy in Act III Sc.1 encapsulates Hamlet's 'tragic flaw' and his moral dilemma.
Hamlet's 'tragic flaw' of course is 'procrastination' - he is well aware that his uncle Claudius has murdered his father and has usurped the throne. Now, all that he has to do is to quickly avenge the evil deeds of his uncle. But Hamlet is hampered by his over scrupulous conscience which prevents him from immediately avenging his father's murder. His conscience prevents him from acting swiftly by debating at length the pros and cons of his action, with the result that the mental process of thinking what exactly he must do completely nullifies the effectiveness of the plan of action he finally decides upon.