The charisma of a great speech, a powerful graphic design or a well-designed tool (and yes, a well-designed tool can have charisma) comes from certainty.
Not the arrogance of, "I am right and you are not," but from the confidence/certainty of, "I need to say it or draw it or present it just this way and I want you to hear it."
Graphic design that fades into the background, that recycles the safe or is merely banal does nothing for us. But the sure hand of someone who understands what she says and what she wants to communicate can't help but touch us.
This is the difference between the mediocre abstract painting at the local crafts fair and the powerful piece at MOMA. This is the difference between 8 bullet points on a slide and a picture that moves us.
Confidence usually implies that you know it's going to work. I'm not talking about that, because only a fool is confident all the time. No, the sure hand can be open and vulnerable and connected, but above all, at least right this moment, it is sure enough to speak up, without hiding.