Everything should be simple, but not too simple, the way it evolved to be in the Nature around us. Things and processes in Nature are as simple as possible, and if something is really complex, there are reasons for it. That complexity usually built up from simple parts.
Simple doesn't mean easy. Keeping it simple is very difficult task. It requires expert knowledge. Simplicity implies clarity and compactness. Less is more when we deal with information. Usually few verses of poetry beats large volumes. Clarity is the evidence.
Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away. This holds in any form of creative writin, especially in software engineering.