Making things subtle is more difficult than making things simple. Simplicity is the reduction of complexity. Subtle goes further, consolidating and predicting future design while still making something intuitive.
Subtle is more complicated because it’s formless. It rises above simplicity and the obscure. It’s hard to pinpoint but just as easy to see and use once you know it’s there.
Everything is simple. Everything is complex. But only few things can be subtle.