By definition, a tragic flaw is that which leads to the protagonist’s downfall. With Juliet, there are a few traits and characteristics that could potentially be said to cause her tragic and untimely end.

You can make a case that her loyalty to Romeo is the flaw that leads to her ultimate demise, as she eventually stabs herself rather than face the future without Romeo. This loyalty to Romeo has Juliet openly defying her parents in time when girls and women were expected to readily obey and accept the commands and wishes of their father, or later their husband. Whether or not they agreed to the decisions made for them were of no consequence. Juliet refuses to accept her father's domination and sets about choosing her own course in life. Her loyalty to Romeo causes Juliet much grief and anguish upon hearing of her cousin Tybalt's death at Romeo's hands. She ultimately decides to grieve only for Romeo, and even berates herself for initially doubting the nobility and honor of Romeo when first she heard of Tybalt's death. Her loyalty to Romeo leads her to also sever all ties with her family and friends. At the end of Act 3, Scene 5, upon hearing the Nurse advise her to forget Romeo and marry County Paris, Juliet responds with "Thou and my bosom henceforth shall be twain."

Another trait that can be considered Juliet's tragic flaw is her impetuosity. Do not forget that it is Juliet her first professes her love for Romeo. Granted, she was not aware of Romeo's audience, but once she is, she does not take it back. It is Juliet who first broaches the topic of marriage and proposes to Romeo at the end of Act 2, Scene 2. Upon hearing that her father has betrothed her to Paris, Juliet immediately flies into a rage, defying her father. She is so distraught after, she rushes to Friar Laurence to beg his help. She does not even give the friar time to fully explain his plan before grasping the potion that will imitate death. It is obvious at that point that she is not considering the possible outcomes. Even later, when she does voice concern over the various possibilities before drinking the draught, she merely voices the concerns, she does not dwell on them. And finally, after waking to discover that Romeo is dead, she does not hesitate to snatch up his dagger and end her own life.

Most of Shakespeare's plays follow a general formula. Something has happened in the society of the characters to cause some turmoil or a major upheaval. In order for things to get back to normal, something has to happen to "shift" the world back. In Shakespeare's comedies, this is accomplished with a marriage or some type of reunion. In the tragedies, the shift depends upon the death of the protagonist. Romeo and Juliet is no different.  But, what has the world off-kilter in the first place? It could...