These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Darkness-Induced Audience Apathy: As with many movies about hardcore crime, some viewers who look for emotional attachments are unable to empathize with the characters and their conflicts.
It being Las Vegas in the 1970s, there's plenty more to go around.
Good Bad Bugs: Rothstein is saved from a car bomb due to a balancing problem in his Cadillac that had necessitated a heavy steel plate being installed under the driver's seat.
Hilarious in Hindsight: Sam makes a very big deal about Nicky being put in the "Black Book" and sees it as evidence that Nicky is out of control. In 1988, Frank Rosenthal, Sam's real life counterpart, was put in the Black Book as well.
This is possibly alluded to in the film, in a subtle way — notice how in the final scene of the movie, set in what is presumably the present day (or at least a few years after the events of the movie), an older Sam Rothstein is now based in San Diego rather than his beloved Las Vegas.
Ginger puts a hit out on Ace and leaves after tying her daughter to her bed.
Frank Marino and the mob bosses crosses it when kills Nicky in a worst sadistic ways possible, forced to watch his brother being beat and killed, then hit him until he is dying and covered in blood, then buried alive with his dead brother. By far, this Moral Event Horizon has gone down in film history as one of the worst acts of cruelty seen in a movie (next to Saw or Hostel).
Narm: The moment where dour old Ace suddenly falls in love with Ginger (though his stony-faced expression belies this), the film freeze frames then goes into slow motion as the rather cheesy "Love is Strange" by Mickey & Sylvia plays.
Narm Charm: Amusement can be also be derived from some of squeaky New Yoiker Joe Pesci's sweary bits ("You muddafucker, you!", "Peek-a-boo, you fucks, you!"), some of De Niro's trademark repetition ("Was I at that dinner? Was I at that dinner?"), one bit of classic James Woods shouting ("Why don't you do it yourself, ya chickenshit cocksucker!") and even some of the violence Crosses the Line Twice, particularly whenever Pesci hits people with phones ("What are you staring at, you bald headed Jew prick?")
Eye Scream The vice scene and bat scene. One because of Nicky, and the other is done to Nicky.
Scorsese reportedly said the vice scene was his "sacrificial" scene, which he would consent to take out if he was threatened with an NC-17 rating. They complained about some other scenes, but not that one, so it stayed in.
Not to mention getting beaten to near death by baseball bats, and then getting buried. Alive.
Special Effects Failure: In the first scene, Robert De Niro conspicuously transforms into a mannequin wearing the same suit right before his car explodes.
Vindicated by Cable: Casino was too similar to Goodfellas during its theatrical release and was dismissed as a lazy attempt by Scorsese. It has grown in stature as time as passed, and some consider it even better than Goodfellas.
Mostly because Robert De Niro's Sam is kind of a better protagonist/narrator than Ray Liotta's Henry Hill (though YMMV, obviously).
What an Idiot: One wonders why Ace didn't put Ward in a place disconnected from the casino floor, like the restaurant.
Ace was freaking out over the blueberries in the muffins. His perfectionism about everything combined with Ward's overarching idiocy is just too incompatible to work at any level. Pat Webb even asks if Ward could have a job "further down the trough" and Ace immediately shoots it down, citing Ward as just too untrustworthy.
In fact, Ward's uncle even asks about a job in the food service area of the casino. Ace pauses for a moment, and the look on his face is practically a Call Back to his perfectionism as displayed by the blueberry incident before he says that no, that's not possible.
Ace marrying Ginger and starting a family with her, even though he knew perfectly well that she was trouble and she told him straight-up that she didn't love him.
He sincerely believed that by showering her with love and affection (and material gains) that she would grow to love him. He just underestimated how dedicated she was to screwing up her own life.
Ginger herself. Everything she does. Ever. Probably the biggest one is anything to do with Amy. Even Nicky explains to Ginger that perhaps the only thing that could ever enrage Ace enough to actually hurt her is taking Amy (who Ginger clearly doesn't care about much anyway). And yet she constantly keeps hitting that button, including when her threats to take the money and jewels provoke little more than apathy from Ace, she tries to get a rise out of him by saying she's going to take Amy.
When Ginger has her Villainous Breakdown towards the end and has a very loud public freak-out on the steps of the house, Amy is watching from a neighbor's house as the whole street looks upon the self-destruction of her mom. And the night before, her mom tied her to the bed just so she could go out to the Leaning Tower and have a drink. That poor kid is going to be for a great time in a psychiatric hospital.
Billy Sherbert, Played Against Type by caustic comedian Don Rickles. There's very little indication that he has much involvement with the criminal activities beyond working as the apparent manager of the casino and generally doesn't do anything bad. Nevertheless, he still has to take abuse from Nicky and get sucked into the insanity of the Ace/Ginger/Nicky situation.