Ultimately, a mafia film about mafia men's attempts to colonize an unfamiliar environment Casino is less of a film about casinos and gambling and more of a film about two mafia men's very different attempts to build a new life in Las Vegas or build a foothold mafia colony in Nevada.
The casino and the gambling industry is simply a backdrop to the story - the film could've been about any other industry with little changes to the main story, which is the two men, almost completely disconnected from the mafia bosses, who run things their own way in a bizarro-land.
It is a bizarro-land in that what's illegal is legal (gambling and bookies are legal), the businesses (casino houses) are very powerful and have their own muscle, there is no mafia presence (other than themselves), no street crime, no rival families or gangs, law enforcement is not as organized, a different kind of government/way of doing things/getting permits and licenses that they're used to, and how they adjust to this completely different environment.
If one were to group mafia films into categories: beginnings of an organized crime family, rise/decline of a family, rise/decline of an individual member/boss of the family, wars between established families, and so on, this would be its own category of mafiosos in a completely unfamiliar environment.
The film is long, but doesn't feel long. It's packed with story - the scenes are short, dialogue is to the point, it is filled with events. It is a long story condensed and cut down to almost 3 hours rather than a short story stretched with long panoramas, filler, story threads that go nowhere, mood scenes and such. There is no fat in the film and the story to footage ratio is almost 1:1 - you can't cut anything without affecting the story.
It's a great film, not just for its time, but for all time. It's not the most original film, but how original can a mafia film be nowadays? It's not high on originality, but it's high on quality.