Unbelievable writing
I must say, this is definitely one of the most excellently written screenplays of all time. The first time I saw it (out of about 127 now), I was blown away. I still get chills sometimes when I watch it.
I can't say it is quite up there with Chinatown, Casablanca, The Godfather I and II, or Sunset Blvd...but it's probably sixth on my list. Impecable by any standards, truly a classic in American cinema history.