Good design is when you see and feel that the work was done properly. “Properly” may be an old-fashioned word, yet it describes best what I consider good design: the result of, and the process of applying the old-fashioned work-ethic of careful craftsmanship to the creation of industrially produced (or post-industrially, like virtual or 3D-printed) artefacts.
Bad design is almost always the result of a thoughtlessness that unfortunately has become all too common. That’s why small details give away bad design so quickly. Meanwhile, the longer you consider the tiny details of a well-designed artefact, the stronger your appreciation becomes for the thinking, problem anticipation and problem solving that went into (and ultimately is) the design.