Ok, I am going to put my two pence into this, and think of it what you will. I will warn you of this, it will most likely be a long, boring post, with examples of how I was.
Now, first thing that I feel the need to say: There is nothing wrong with being quiet. It is a personality type, just like being loud, controlling or submissive. Now, if you want to change, to get out there and discover what people really think about you, I would advise that you get a job where you are forced to communicate. I used to be introverted to quite an extreme, and then I got a job as a waiter, and needed to, not only speak, but know when to be nice, and when to be cutting (depending on whether or not you are in the kitchen).
Now, as for why quiet ones are the ones to look out for, my opinion is this: They listen. I am a quiet person, and I find that people often have conversations with me in the vicinity, but seem to forget that I am there. I have no problem with this, as, if I wanted to make myself heard, I would, and I learn no end of things that could either hurt others, or help me (e.g. picking conversation subjects, picking shifts). If I actually became vindictive, I have enough ammunition to hurt just about everyone I know.
BUT, I would really advise that you have an outlet for anger and rage (and yes, no matter how peaceful a person you are, you still need it), whether it be video games, sports, or (in my case) shouting at young children who really annoy you (lifeguard, it definately has it's perks).
Anyway, that is my opinion, do with it what you will