Ikat'ika: Victory is life. Worf: Today is a good day to die.
Batman Gambit: The Dominion anticipated that the Federation and Klingons would make a stand at the station, which is why they plotted to destroy Bajor's star. The resulting shockwave would cripple both fleets, as well as destroy the station and leave the wormhole up for grabs.
Brick Joke: Jadzia kept Worf's opera collection intact...more or less.
Worf absolutely refuses to give up the fight against the Jem'Hadar First, even though he can barely stand up. This earns the First's respect.
Garak does his best to press on with his work in the cramped space in spite of his claustrophobia, until he suffers a crippling panic attack when his jury-rigged light goes out. Once he's recovered enough, he goes right back to work.
Didn't See That Coming: A fleet of Romulan ships de-cloak around the station and requests permission to join the fleet.
Sisko: I'll be damned. Permission granted.
Dude, Where's My Respect?: Dukat demands that Sisko show him more respect as the head of the Cardassian government. Sisko tells him to stuff it.
Enemy Mine: The Klingons and Romulans put their problems with the Federation aside to join against the Cardassian-Dominion threat. The Klingons even re-sign the old Khitomer accords.
Not to mention all the prisoners in the compound to escape.
Evil Is Petty: Because the Cardassians are now allies with the Dominion, all the Cardassian prisoners are freed. However, Dukat explicitly forbids Garak from being one of them.
Face Your Fears: Garak faces his severe claustrophobia to finish his work on the transmitter. Even Worf and Martok are impressed.
Martok: There is no greater enemy than one's own fears.
Worf: It takes a brave man to face them.
Famed In-Story: Martok promises that Worf will have a song written about his fight against the Jem'Hadar. He also offers to include Bashir, "the healer who bound the warrior's wounds so he could fight again!" Garak then points out that the Cardassian who panicked in the face of danger would ruin Martok's heroic epic, and resolves to finish the job he started in spite of his claustrophobia.
Godzilla Threshold: Going to warp inside a solar system is portrayed as a dangerous maneuver, but Kira points out that if they don't, there won't be a solar system left. It's worth noting that they're warping directly toward a star, so a slight miscalculation would equal one crispy Defiant (and probably wouldn't be very good for the star either, not to mention the planets orbiting said star).
Gondor Calls for Aid: Sisko convinces Gowron to rejoin the Khitomer Accords, allowing the Klingons to join Starfleet for the defense of the wormhole. Surprisingly, an entire Romulan fleet also joins in to help their foremost enemies.
Honor Before Reason: Why Worf continues to fight a losing battle, even after Ikat'ika (his opponent) and Martok (another Klingon) tell him to give up.
Martok: Worf, honor has been satisfied. Stay down.
Also discussed before the fight begins:
Deyos: You fight because that is what you were designed to do. All that motivates him is some barbaric sense of honor. Ikat'ika: And that is something you will never understand.
Hope Spot: The Vorta announces that Cardassia has joined the Dominion and so all Cardassian prisoners are going home. Garak starts to go with the others, until it turns out that Dukat intervened personally to ensure he would not be released.
I Owe You My Life: Dukat freely admits that Sisko saved his life at least once, and returns the favor by giving him a chance to surrender peacefully.
Mutual Kill: The Breen prisoner and a Jem'Hadar guard vaporize each other.
Neck Snap: How Worf kills one of his Jem'Hadar opponents.
New Era Speech: Dukat delivers one after he removes the Cardassian civilian government from power.
"You might ask, should we fear joining the Dominion? And I answer you, not in the least. We should embrace the opportunity. The Dominion recognises us for what we are: the true leaders of the Alpha Quadrant. And now that we are joined together, equal partners in all endeavours, the only people with anything to fear will be our enemies. My oldest son's birthday is in five days. To him and to Cardassians everywhere, I make the following pledge. By the time his birthday dawns, there will not be a single Klingon alive inside Cardassian territory or a single Maquis colony left within our borders. Cardassia will be made whole. All that we have lost will be ours again, and anyone who stands in our way will be destroyed. This I vow with my life's blood. For my son, for all our sons."
Oh, Crap!: The Bashir changeling has this look when his runabout gets tractor-beamed short of his target.
Plot Hole: There is no reason for the Runabout to be in orbit other than so the protagonists can make their escape. This was later cited in "Inquisition" as implausible and "proof" of Bashir being a Dominion spy.
Sadistic Choice: Dukat tells Sisko to either surrender the station or that he'll retake it. Sisko basically tells him to Bring It.
See the Whites of Their Eyes: Used and averted. Used in that the Defiant has to get very close to the Yukon in order to use the tractor beam on it. Averted in that they were shooting the Yukon from far enough away that they had to make a brief jump to warp to get close enough to tractor beam it.
Sensor Suspense: With what looks like a fleet of Dominion ships. Turns out to be a distraction while the changelings carry out their real plan.
Spot the Imposter: Discussed at the end of the episode, where O'Brien is stunned to learn he spent over a month associating with a Changeling.
O'Brien: And the worst part is, the clues were right in front of me. Bashir: What clues? O'Brien: Well, for one thing, he was a lot easier to get along with.
Star Killing: The Dominion tries to destroy Bajor's sun with a protomatter/trilithium device.
Taking You with Me: The Breen prisoner kills a Jem'Hadar guard in a sneak attack, then the prisoner and a second guard kill each other.
Female Romulan: My people have a saying: "Never turn your back on a Breen."
Wham Episode: Ron Moore described this two-parter as "throwing up everything into the air, and only what they caught would stay the same. Everything else, not so much."
Dukat joins the Dominion, installing himself as the leader of Cardassia and giving the Founders a foothold in the Alpha Quadrant.
Gowron rejoins the Khitomer Accords, making the Federation and Klingons allies again.
Wham Line: Courtesy of Dukat before the teaser is finished.
"I'm not attacking the Dominion fleet, Major. I'm joining it."
The Worf Effect: Played with. Worf defeats several Jem'Hadar in a row (while suffering more and more injuries), but eventually he ends up facing Ikat'ika, who wipes the floor with him. Worf's Determinator spirit, however, remains unbroken, and in the end it's Ikat'ika who yields.
Worf Had the Flu: By the time Worf fights the First of the Jem'Hadar on the station, Worf has already experienced progressive injuries from his previous 7+ opponents, which Bashir can only do so much about under the circumstances.
Worthy Opponent: Worf kicks several different kinds of ass during his forced fights on the prison asteroid. His sheer determination earns him the respect of the Jem'Hadar in charge.
Ikat'ika: I yield. I cannot defeat this Klingon. All I can do is kill him. And that no longer holds my interest.
You Have Failed Me: The Vorta Deyos has Ikat'ika killed for yielding to an opponent he was winning the fight against. He also tries to have Worf killed at the same time, but Worf gets beamed out at that moment.