Great is easy to tell,it is forward,
simple to the point, and makes a difference. It has great technique and tells an amazing story. In both aspects: basic & iconic.
But where lies the line between average and mediocre? Is it execution, technique, the forwardness, the story? That the basics work? An amazing parking lot scheme? Making money? How "easy" it was to build? How timeless or fashionable it was sold as? Who made it? The greenness? The facade? How many people can recognize it? The embodiment of "what people want"?
Simple answer: great design, avoid cheapness, get in the contemporary mindset, basics are beyond good, avoid odors, let the humans breath, and to the very least go somewhere beyond basics, clichés, gimmicky "pluses", and pretentious
Basics are the poverty trap, if you are only doing the basics, if you can't see the panorama beyond the details of bolts, bathrooms and parking lots, YOU are mediocre.