The aforementioned violence, obviously. Also, when Henry gives Karen a huge wad of shopping money, and she immediately drops to her knees, and his response is, "Oh, okay..."
"You mean funny like I'm a clown, I amuse you?"
"I almost had him!"
"You really are a funny guy!"
"OH, THAT'S IT, HENRY!"
Jimmy and Henry are in Morrie's wig shop, trying to get back some money they lent him while a commercial he made (apparently with said money) is playing on a continuous loop boasting about how his wigs never come off. While Henry pleads with the poor guy, Jimmy stands back and quietly watches the commercial, slowly getting more and more pissed off. Finally he explodes and begins to strangle Morrie with the telephone cord.... causing his wig to fall off! Henry bursts out laughing.
At his son's birthday party Jimmy, the hitman, thief and extortionist, is wearing little conical birthday cap. Better yet he's the only adult wearing one.
Jimmy is scolding a fellow thug for buying a cool car just after the big heist, even though Jimmy explicitly told everyone not to buy anything big. The scene is pretty tense, until another thug comes in with his girlfriend, who is wearing a very attention-grabbing fur coat. De Niro's reaction is priceless.