A masterwork is a highly accomplished game that touches both play brain and art brain, fascination and numina, and so most players consider it a thaumatic experience. Masterworks do not need to be founderworks: they do not need to feature a ground-breaking innovation.
A key feature of masterworks is sophistication. Market niches and tribes respond to them because they use signifiers and more complex communication to speak to the audience, carrying an aesthetic as well as a literal meaning for them. As with Renaissance artwork, the work is layered for the audience that understands the layering, and impressive (or even amazing) for those who don't.
Games are leaving their founderwork age behind and entering their masterwork age.