As for “revolutionary” techniques, the film is remarkable for how extensively it relied on existing special effects techniques, and how well it masked their use. There are numerous shots in the film which are composites of three, four or five matte shots.
Sometimes the results are palpably phony; when the camera zooms in on the nightclub where Susan Alexander is interviewed, you can readily tell that the building is just a model. But one is not likely to realize that the sculpture of Thatcher in the lobby of his library is just a little table top-size model, “set” on a pillar which didn’t exist. Similarly, remarkably little of the interior of Xanaduu was real; a matte shot of a hallway would be placed inside a door frame surrounded by a painting of a wall and matte shots of tiny statues which had been blown up. I have read that the bats which appear at the “picnic” scene are actually footage from Son of Kong. And while it is just a small thing, I for, for one, can’t tell that the trucks building Kane’s mansion are really just ordinary toys.
Another remarkable example of artifice which goes unnoticed; in the long shot where the camera trucks upward past the opera stage to the stagehands standing above, there is actually a huge cut in the middle; this was two shots edited together, so that they matched seamlessly.
The film hardly invented nonlinear story telling. It has been noted, for instance, that the overall structure resembles a Spencer Tracy movie written by Preston Sturges called The Power and the Glory. Nevertheless, it had a complexity of structure which was pretty well unprecedented in a feature film.
Prior to its release, Orson Welles remarked in interviews that the movie was “a new kind of film”. He seemed to be referring largely to the complexity of Kane’s character, and the detachment the film has in examining it. While there had been movies about anti-heroes before–Warner Brothers gangster films in the 30s come readily to mind–they had tended to be fairly simplistic in depicting the central character. As Welles observed in the coming attractions trailer, Kane was both a great guy and a dirty bum.
Of course it is worth noting too that Welles gave a superb performance, depicting a complex man in many moods over a period of something like sixty years, and that the film featured the first, and arguably best, screen performances of a number of other distinguished actors.
Is it the greatest film ever made? IMHO it is silly to insist that there must be one greatest film that everyone can agree on, and silly to claim that one has seen enough films that you can be absolutely sure which movie it is that’s best. Nevertheless, Citizen Kane must be counted as one of the outstanding films of all time.