Coitus Uninterruptus: In life, Garcin was fond of bringing his conquests home in full view of his wife, and making her serve them coffee in bed. He does it with Estelle during the course of the play whilst Ines offers running commentary, in a supremely awkward scene.
Freudian Trio: Garcin is Ego, the mostly critical Ines eventually becomes the Superego (while acting like Id at first), and Estelle reveals herself to be Id.
Hell Hotel: Arguably, this is the inversion (rather than an Earthly hotel being hellish, Hell is a rather normal Earthly hotel—on the surface anyway).
Ironic Hell: Possibly the most stripped-down, bare-bones example in the history of fiction. The only things making the hotel room a place of eternal torment are the exact same psychological flaws and unpleasantnesses that got you sent there in the first place.
Jerkass: The three of them in life, which comes to be used as their mutual hell.
Only Sane Man: Garcin believes himself to be this, being a provider, and a pacifist, and therefore has no reason to be there. He's neither. When he and Estelle begin lying, painting themselves as better people, and the fact that there must be a mistake, Ines sees through them, and accuses them, and herself, of being all rotten people, who should just admit what they did. She's also the first to hammer in the fact that they're in hell, and deserve to be there.
Self-Inflicted Hell: Literally, there is no need for Fire and Brimstone Hell because "Hell is other people". Made even more poignant by the fact that the protagonists are given several opportunities to escape during the play, but are held back every time by their own flaws, fears and anxieties. Not only is hell other people, but its security system is yourself.
Silly Rabbit, Cynicism Is for Losers!: This is part of the entire point. At the end of the story, it's implied that the main characters could leave at any time they wished to, but their own character flaws and lack of empathy with each other prevent them from doing so.
Triang Relations: Inverted, subverted, turned inside out and dumped all over the floor. Hell picks the roommates very well; every one of them is CAPABLE of respecting each other at least superficially. But when any two start getting along, the third person will try to break it up out of jealousy; all three are attention whores and cannot stand the other two being friendly. Estelle causes problems by making romantic overtures at both of them, Ines by picking one to degrade in the eyes of the other, and Garcin by trying to make himself look superior to one or the other. The three are perfectly suited to constantly making each other miserable via social shenanigans, and yet incapable of dealing with the idea of leaving the other two.
Write Who You Know: Garcin is based on the author, Jean-Paul Sartre. Ines is based on his long-term significant other, feminist Simone de Beauvoir.
Yandere: Estelle was displeased by one of her lovers, so she took a horrible revenge by throwing their child off a balcony in front of him.
Year Outside, Hour Inside: During the play, the characters have visions of life progressing without them on Earth, and it seems like unlike usual Hells in which an eternity is actually a second, what feels like a brief time in Hell is actually several months on the outside.