One of Juliet's primary traits is her ever-present impatience. In part, perhaps, because of her age, Juliet seems incapable of allowing events to develop at their own pace and, instead, consistently tries to force various issues throughout the play.
For example, in Act IV, scene i, when Juliet discusses Romeo's banishment with Friar Lawrence, she states:
Give me some present counsel, or, behold,
'Twixt my extremes and me this bloody knife
Shall play the umpire, arbitrating that
Which the commission of thy years and art
Could to no issue of true honour bring.
Be not so long to speak; I long to die,
We see here Juliet's impatience and her eagerness to take matters not only into her own hands but also to extremes.
In fact, Juliet comments on her own impatience during her soliloquy in Act III, scene ii:
To me, Juliet's major character trait is that she's fourteen. I know, that's not a character trait, but it leads to one. I think her major character trait is that she is overdramatic and gets too caught up in her new love. I think that if either she or Romeo had been a bit more mature, things could have turned out a lot better. Here are a couple of quotes that could support this:
The first is from Act I, Scene 5. Romeo has just left and Juliet is head over heels. She says:
Sounds dramatic, huh? If he's married, I'm totally going to die, OMG!!
Another one could be from Act II, Scene 5. She's sent the nurse off to talk to Romeo. She's back and she's all complaining that she's tired and sore. But Juliet can't wait. She says, for example
I' faith, I am sorry that thou art not well.
Sweet, sweet, sweet nurse, tell me, what says my love?
Yeah, yeah, I know you're tired. So what did Romeo say??