Outstanding performances
Everyone (at least everyone who likes this movie) can agree that Warren Beatty, Julie Christie and Goldie Hawn all turn in fantastic performances.
But even the smaller roles feature amazing performances, too. Jack Warden, Lee Grant, Tony Bill, and even the cameos by Howard Hesseman and Carrie Fisher are all GREAT!
I guess it all started with a great script by a writer (Robert Towne) at the top of his game, combined with a great director (Hal Ashby) who was equally on fire. Combine that with excellent casting, and you've got some fantastic performances.