Good design, like good interface, should not be obvious. It should be subtle, and yet be strong enough to get the message intended across to the person interacting with it.
Interface, considered by many to be a subset of design, is in my mind is even more subtle and pervasive.
We interface with everything every moment of every day. And humans have been doing so for a very long time, even before design was perfected to be borne as a profession.
As many are familiar, an interface is known as an User Interface (UI) on a display to interact with to move things around, show pictures or movies and read text. In other words control the workspace. However, that is just the "virtual space". The human interface is the computer input: most notably the keyboard and mouse to most. While computers have other means of interfacing with the device (speech, eye-tracking, brain-wave), these are not as well refined or implemented at this time.
Today's computers are simply complex tools. Tools of older generations of humans had tools of wood, stone, etc. to interact with their world. Tools come in many forms, including: pencil and paper, fork and knife, water in a glass, chest of drawers with clothes, gas stove to cook food, lock and key, etc. Each a technology at the time invented. Collectively, also each an interface.
Architecture is one of the best human interfaces ever created. People interact with the spaces they live and work and gather in. Well architected interfaces last many decades or even centuries. With good construction materials, even millenniums. Well done things tend to be kept in use.
A book is a great interface that has lasted centuries. It contains information in text and sometimes pictures and/or diagrams. It is portable, can be easily transferred, and has the ability to become a historical item when it survives for a long time (and thus an indirect interface to that time period).
Good signage, a must to inform someone in an unfamiliar territory or provide a warning, is also an interface. It communicates to the person a message.
That signage is really useful when in a vehicle, an interface to move you faster than walking.
In short, anything that interacts with something else is an interface. Even the mind interacts with the human body to interact with the "outside" world.