This episode builds on the humanization of the character of Gul Dukat which began in "The Maquis, Part II" from the second season and continued in the third season episode "Civil Defense". In "Defiant", Dukat is saddened about missing his son's birthday, and his interpretation of what that will mean to his son in future years is uncharacteristically honest. This softening of his character continues up to the fifth season episode "By Inferno's Light".
It is unknown if William T. Riker visited Quark's during the Enterprise's first visit to DS9 in "Emissary", its second visit in "Birthright, Part I" or another time altogether, though an off-hand comment by Jadzia places his visit closer to that of "Birthright, Part I" ("I met him a year ago"; discussing Riker with Kira).
The large Cardassian viewscreen seen many times in this episode is actually a miniature designed by Gary Hutzel and built by Tony Meininger. The shots of Sisko standing in front of the screen were composited shots of the miniature and actor Avery Brooks in front of a bluescreen.
Quantum torpedoes are used for the first time by a Federation starship in this episode. The Cardassian Union had quantum torpedo technology as early as stardate 47582 (mid-2370) as Cardassian ATR-4107 was equipped with these weapons.
This episode features three 47 references (or just one if you understand how numbers work): The Defiant features the registry number NX-74205 (which is not a 47 reference), Sisko's commander's log mentions the stardate 48467 (which is not a 47 reference), and the Cardassian Outpost 47 is mentioned in dialogue.
The trivia items below may give away important plot points.
Although this episode was screened three days after the release of Star Trek Generations, the stardates indicate that it takes place shortly before. This makes sense, as Riker and Sisko's conversation would seem to indicate the Enterprise is still active.
This episode introduces a fleet of Keldon-class ships secretly constructed by the Obsidian Order in the Orias system. The later third season episode "Improbable Cause" reveals the reason for their construction: they were to be used in a joint operation by the Obsidian Order and the Romulan Tal Shiar to launch an assault on the Founders' homeworld (precipitating the Battle of the Omarion Nebula).