It’s a masterclass in acting, cinematography, and soundtracking. Not only that, but Mario Puzo’s novel which came out shortly before the movie was produced was a smash hit.
In the 70s, movies didn’t look like The Godfather. They looked like weird objective cameras put on a tripod and just filming actors, with not as much thought put into the “feel” of the film. FFC (as well as other directors such as Hitchcock and Kubrick) essentially invented modern cinematography. Remember watching Avatar for the first time? It was kind of like that for movie going audiences.
It was always hailed as an “epic drama” so you have to kind of temper your expectations based off that. It’s not a “murder a minute” gangster flick like a Scorsese picture.