I would add that aggressive drivers act impatiently. They are not content to be in the flow of traffic and watch out for other drivers, even assist them in finding space for a lane change. But, aggressive drivers are constantly trying to beat someone else out and get ahead, even if the only advantage is a little more time at the next red light.

The "I know it when I see it." quotation comes from Supreme Court Justice Potter Stewart in a 1964 court case on hard core pornography. (I googled it.)