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.
Perhaps that is why we love those small indispensible graphical images so much, it’s an appreciation of the hard work, creativity and talent of the designer. And to think, all those hours spent in front of a laptop crafting these little marvels, and then, allowing everyone else to download them for free. To all icon designers we say: “thank you”.