They show initiative. They do things they have not been told to do. It may be exceptional intelligence and insight. It may be unusual creativity, organizational skills, commitment - or a some combination. These gifts are evident to the teacher and usually to the other students as well.
Attention is paid to conciseness and completeness.
Their work is a pleasure to grade. They put other priorities ahead of academic work. In some cases, their health or constant fatigue renders them physically unable to keep up with the demands of high-level performance. Their work may be sloppy or careless. At times, it is incomplete or late.