Hamlet's tragic flaw is that he is indecisive. This indecisiveness seems to be related to the fact that he is a deep thinker, and this in turn seems to be related to the fact that he has been a scholar for many years and has kept himself buried in his books. Although it is established in the so-called gravedigger scene that Hamlet is thirty years old, he was attending Wittenberg University when his father died and wishes to return there after having attended the funeral of his father, the coronation of his Uncle Claudius, and the marriage of his mother to Claudius. However, his uncle forbids it. It would appear that Hamlet would like to study for the rest of his life and that he is appalled by the obligation imposed by his father's ghost to take revenge against Claudius for murder.
The play is noted for being full of Hamlet's melancholy soliloquys. Instead of taking action, he gets lost in his own thoughts, which he expresses aloud although they are intended to be understood as his stream of consciousness. His deep thinking does not prevent him from acting, but it prevents him from feeling the rage he needs to feel in order to murder his uncle. C. G. Jung points out in his Psychological Types that thinking and feeling are antithetical. This would seem to be the essence of Hamlelt's tragic flaw.