The Defiant-class was constructed and designed at the Utopia Planitia Shipyards and Commander Benjamin Sisko was part of the design team. Construction was slow due in part to a perceived lack of an imminent Borg attack and to engine design problems that plagued the starship even after construction was completed. The engines were so over-powered that they threatened the very structural integrity of the ship when powered up. It took innovations in the structural integrity field of the ship to bring this problem under control.
Armamentpulse phaser cannons that apparently provide more fire power than the standard phaser arrays on most starships. These weapons -- along with photon torpedo launchers (later able to fire quantum torpedoes) -- give the ship a very powerful forward weapons arc. As Sisko stated succinctly, "She's overpowered and overgunned".
The Defiant-class also has experimental ablative armor that gives the little ship an extra edge if its shields are brought down, as the armor provides extra protection not usually afforded to Federation starships. The armor's effectiveness was demonstrated when the Defiant withstood concentrated attacks from a Klingon Vor'cha-class attack cruiser for several seconds without shields in order to beam aboard members of the Cardassian Detaapa Council and when the Captain of the USS Lakota complained about the ablative armor on the Defiant making it difficult to disable or destroy her. It should be noted that the captain also stated that this armor was a secret modification of Captain Sisko's, which opens the question of whether the ablative armor is actually unique to the USS Defiant and not standard for the class.
Other unique features of the Defiant-class include her warp engines, which were kept in close and parallel to the hull, as well as the spartan crew quarters and overall interior design of the ship. Doctor Bashir complained about the bare medical bay that was inadequate to deal with serious injuries as well as the lack of any sort of laboratory facilities found in other starships. No families could travel on the Defiant, and her crew complement was normally kept around 47.
The Defiant-class proved time and again she was more than a match for substantially larger vessels. During the Klingon-Cardassian War she routinely defeated Birds of Prey in combat and withstood attacks from multiple Klingon warships many times her size, such as Vor'cha-class attack cruisers. When stolen by Thomas Riker, the Defiant also proved more than capable of defeating Cardassian capital ships, such as the Keldon-class destroyers secretly upgraded by the Obsidian Order.
The Defiant-class also performed admirably in her first encounter with the enemy she was designed to battle -- the Borg -- when she was among the surviving vessels of the Starfleet task force that fought a running battle over several days from the Typhon sector to Earth against a Borg Cube. Scoring damaging hits against the far larger Borg cube, she was finally disabled by a concentrated Borg counter attack after out-lasting far-larger starships.
During the Dominion War, Defiant-class starships were usually seen as more than a match for the Jem'Hadar fighters and only truly outgunned and outclassed by such juggernaughts as the Dominion Battleships. The Defiant was the first Federation starship to score kills against Dominion naval forces, and ships of the class were instrumental in almost every major engagement of the war.
The Defiant-class in Debates
Rivalry between Trekkies and Fivers resulted in numerous debates over the possible outcome of a battle between the the White Star and the Defiant. Trekkies typically argue that the Minbari stealth technology would be no match for Federation sensors, while a mere proximity blast from a photon torpedo would be more than sufficient to disable a White Star.