As Saint Exupery said,
A designer knows he has achieved perfection not when there is nothing left to add, but when there is nothing left to take away.
And Joshua Porter adds:
Simplicity is treading a line: knowing what to keep and what to throw away. It comes across as magic when it works, because none of the complexity is transferred to users, only simplicity.
Nothing to add, really (emphasis mine)
(from Five Principles to Design By)