First – No turn indicators. Come on, it isn’t that hard to let people know where you are going.
Second or could be first – People who stop short. I see this daily. As a defensive driver you are checking mirrors and looking ahead. You are calculating where and when to move if it is your choice or made by others. Then you have people who stop hundreds of feet from a stop light or stop sign to let someone in. Of course it does not help that the idiot from the parking lot started pulling out when he knows there isn’t a chance in hell of cutting across traffic. But he is obliged and makes his move.