As a grandpa who does a lot of babysitting--I have 5 grand kids under the age of 4 1/2 with another on the way--I believe in the theory of positive discipline. Info on this can be had at the sites of Rational Jenn and The Little Things.
But I want to focus on one aspect of discipline: talking to young children when they are behaving badly. I've seen adults yell at their kids saying things like "You're being bad" or You're a bad boy/girl" and "What's wrong with you?" and so on. I am afraid though that a steady diet of words like that might cause the kids to conclude that there is something innately wrong with them over which they have no control. I don't want to take that chance.
Now I like to use different words and phrases. When they are being defiant or otherwise misbehaving, I tell them they are making bad decisions or bad choices and I try to follow that up with reasons why that they will understand. I think this reinforces in their mind the fact of free will while informing that not all choices will be good for them.
Of course it's a law of nature that kids forget fast, so constant repetition is needed. So is lots of patience. This is because their desire to use their free will can be almost irresistible. That can lead to conflicts.
Of course the flip side of this coin is commenting on when they do good. I like to see them get their happiness from the achievement of things without me saying anything. But young kids often want to share a new accomplishment with their adults so when I comment I say things like "Good job" or "Well done" or "Good thinking" and so on. I try to avoid "You're so smart" or 'You're really good" etc.
Positive Discipline has more to say on these subjects so be sure to visit the above links.