In my opinion, this was the first film to establish Robert Deniro's onscreen persona & his distinctive method of acting that he's known for, even up until to this day.
And that's probably more due to the film's director, Martin Scorcese, and his particular brand of directing. Scorcese really seemed to know when to allow Deniro's mannerisms to freight this portrayal of a cab diver whose loneliness in the vast sea of grime that was '70's era New York City was consuming him to the point of razor-edged madness.
It was an acting freedom that Scorcese used to maximum effect and which also allowed the supporting roles of Jodie Foster, Cybil Sheperd, Harvey Keitel, Albert Brooks and Peter Boyle to bring the depth of character that they were each able to contribute to the film (particularly, IMO, Foster and Keitel).
While Martin Scorcese is more famous for his movies that delve into the mafia lifestyle, I feel that his talent for depicting the grit of the streets, particularly of this time-period, cannot be understated andd is what really carries the feel and weight of this story.
Taxi Driver is a true staple in the decade of 70's films, and a piece of high quality and expressive cinema that I really enjoy watching over and over again.
I'm talking to you.