John Ritter's dance number in "Up In the Air" at a cocktail party where Jack is loopy from mixing tranquilizers to calm him on a small plane ride and alcohol at the party. It's a tour-de-force.
When Roper mocks gay people for being "interior decorators", Jack points out that some gays are also boxers. When Roper expresses doubt that a gay person would hit another guy, Jack says "Only if they make fun of us" and cracks his knuckles.
Also a Heartwarming Moment: In Season 6, Jack mistakenly serenades a woman named Bernice instead of his evil blonde girlfriend, Arabella (a stereotypical Southern belle). Bernice gives a speech about their romantic Meet Cute, and it's clear she Thinks Like a Romance Novel. In the next scene, when Arabella makes fun of Bernice, Jack stands up for Bernice and repeats her romantic speech, and then he kicks out Arabella.
One episode has a lonely old man spending time at the apartment, much to the chagrin of a woman trying to score with Jack. Said woman finally snaps at him when the two are alone in the living room, telling him that he's a nuisance and should leave, which he does. When Jack reenters the room, he asks where Leo went:
Nancy: He left.
Jack: He left?
Nancy: I told him he was a nuisance.
Jack:(angrily) You what?!
Nancy: Come on now, who would you rather spend time with? Him, or me?