I would say the variety of it. The job changes constantly- from renovations, like putting in hot tubs and heaters, to working on a high rise using iron materials, to getting rain water from the roof or waste from sewage lines. It s very hard to get bored in this line of work. Doing things right the first time, I would say. You can always do a quick job on something that no one will see, but it will catch up to you later.
Problem solving skills are very good to have, too.
You have to be very inventive sometimes to assess something and know how to get the job done without wasting time or materials.