Design should make something easier to understand, easier to use; it should emphasise the function of the piece. It is often said that good design is invisible, as it is less likely your audience will reward you for a well functioning design, but will criticise the frustrations of a bad one.
Taking great care in every stage of the design process is to have full respect of the piece and the end user. Nothing should be left to chance, the aim should be perfection. Small inaccuracies may escape unnoticed in small doses, but as a whole, their feeling will be felt.