Onur Cobanli explains:
I have expressed this in different forms but the idea is that most designers stop designing once they arrive at a satisfactory good design even if there is still possibility to develop great design; even if there are more space for iterations, refinements and developments most designers stop at good design and never arrive at great design, that is why good design is the enemy of great design; to arrive at great design you need to keep improving beyond the satisfactory good design level; you must leave your local maxima and search for the global maxima; you need to keep searching, iterating, refining, experimenting and improving. Good design is like an obstacle in the path to great design.