The most important aspect of designing an individual icon is that it has to be instantly recognizable, you have to know its function, and you need to know exactly what it is. If you are designing a set, the actual design becomes a lot harder, the designer has to face two initial challenges: each icon they design has to be individual and distinctive from the other, and, they also have to have similar features to show that they are related, completing the set. Not an easy task.
