Citizen Kane is about the life of a newspaper tycoon, who begins his career on grounds of idealism but eventually gets enamored with the smell of success, and leads his newspaper into a very yellow and very popular journalism. His single-minded and overbearing pursuit of money, fame and power leads to his eventual alienation with his friends, lovers and principles. The movie begins with his death and traces his life through interviews with people he interacted with.
Its true that the plot of the movie is not terribly path-breaking. There are a lot of movies which are loosely based on the lonely man at the top. However, Orson's rendition of the theme is nothing short of perfection. He has turned the biography in a mystery, as a newspaper reporter attempts to understand the meaning of Kane's dying word - 'Rosebud', which leads him to reconstruct the tycoon's life.
Then there are several subtle depictions like the breakfast scene between Kane and his first wife Emily. In a run of 4-5 consecutive breakfast scenes, Orson masterfully depicted the changing relation between the spouses. Another subtle message is the camera's focus on 'No trespassing' sign both in the beginning and the end of the movie and the picturization of the palace life with jigsaw puzzles, countless statues and endless mirrors. I particularly loved the shot where Kane is shown in the many mirrors of his palace - nothing could have shaped his isolation and loneliness better.
The movie of course courted many good reviews, and the critics applauded it for its innovation. For the first time a movie used a combination of elements such as newspaper reports, narratives, diary entries and memories to tell its story.
Jorge Luis Borges summarized the movie very succinctly when he called it a
...metaphysical detective story, its subject (both psychological and
allegorical) is the investigation of a mans inner self, through the works he has
wrought, the words he has spoken, the many lives he has ruined.