What Made Gran Turismo 4 One Hell of A Game?

Sony's single longest running franchise at this point (particularly in the wake of the untimely demise of Wipeout) is Gran Turismo. Polyphony Digital's series of superlative racing sims popularized and pioneered the genre, and achieved record high sales around the world. For years, Gran Turismo games were considered to be the peak of the genre, and the ultimate representation of motorsport and the machines that make it possible, in video games.
Every game was a painstakingly and laboriously crafted love letter to automobiles, with a stunning degree of authenticity and attention to detail that was, frankly, absurd (in the best ways possible). The original Gran Turismo set the stage for this, and then its sequels continued to up the ante, becoming bigger and better, and somehow in the process, still more detailed and authentic.