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:
Go ask his name: if he be married.
My grave is like to be my wedding bed.
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??