Deep Space Nine opened to strong reviews and critical acclaim, but the first season was later criticised for being a little too dull. Episodes like Duet and, in the second season, the introduction of the Dominion swiftly remedied such complaints.
Over its first two seasons, DS9 concerned itself primarily with the political-religious situation on Bajor, the antagonistic relationship with the Klingons and, with decreasing frequency, dropping in TNG characters like Q, Vash, Lwaxana Troi and the Duras sisters. Although this was often successful - such as Sisko punching out Q, leaving Q feeling that Sisko was less interesting than Picard and never visiting the station again - it occasionally left the writers feeling like an adjunct to TNG. They spent more time moving away from the TNG template by doing things Star Trek had not done previously, like revisiting previous storylines more frequently, developing a large cast of secondary characters and, at the start of Season 2, creating Star Trek's first-ever three-part episode (later on they would also create Star Trek's first - and only - six and nine-part episodes as well).
Paramount were initially very happy with DS9's performance, but also responded to fan complaints that the new show was a little too stationary by ordering a second spin-off from Rick Berman's team, to their surprise (they'd assumed DS9 would be their primary focus once TNG ended in May 1994). Rick Berman and Michael Piller were soon distracted creating the new series, Star Trek: Voyager, and they needed a new team to step up and take command of Deep Space Nine.
In the second season DS9 began downplaying the Bajoran-Cardassian storyline, noting that some fans were not keen on it, and started developing its own set of villains. They didn't want another monstrous foe like the Borg or yet another race obsessed with honour. After casting around for ideas they hit on the notion of an "Anti-Federation", a coalition of disparate races in the Gamma Quadrant who had joined together under the leadership of the persecuted shapeshifters to defend themselves and, over time, had became the despotic, tyrannical power they had originally set out to avoid. The Dominion, as they became known, were slowly teased and unveiled over the course of the second season before making a full impact in the enormously popular second season finale, The Jem'Hadar, where they destroy the Galaxy-class USS Odyssey as a show of might.
The introduction of the USS Defiant, the Federation's first warship, was mildly controversial.
For the third season fresh creative blood came on board, most notably experienced TNG writer Ronald D. Moore. Moore had just got off the boat (metaphorically) from finishing up TNG and co-writing Star Trek: Generations. His first assignment was to give DS9 a powerful new warship that would help defend it from the Dominion, the USS Defiant (aka "the tough little ship"). However, the biggest change as the season progressed was Michael Piller moving over to Star Trek: Voyager and Ira Steven Behr stepping up as showrunner.
Behr was a much more character-focused writer than his Star Trek forebears. When he took over, a series of informal rules were put in place: no holodeck malfunction episodes and as little technobabble as possible. He wanted stories rooted in the characters that weren't afraid to be a bit more morally murky and questionable than Star Trek had been in the past. His team delivered in spades and the cast responded enthusiastically to being given more challenging material, particularly Avery Brooks who was proud that the show was prepared to tackle issues such as racism, discrimination, homelessness and religion.
Lt. Commander Worf (Michael Dorn) arguably got more character development in four seasons on DS9 than he did in seven seasons of TNG.
Behr also knew how to play politics. Despite his - by Star Trek standards - radical ideas he maintained the trust and support of Berman and Piller, who eventually left DS9 unmolested to do its thing whilst they focused on Voyager. In fact, Voyager got the lion's share of publicity and marketing as Paramount used it to launch their new TV network, UPN (which later merged with The WB to become The CW), but Deep Space Nine soon developed a knack of stealing its thunder. The arrival of Michael Dorn (Lt. Commander Worf) on DS9 and the development of a season-long arc about renewed hostilities between the Federation and Klingons led to a boost in ratings and critical acclaim. Voyager had a nice, small-scaled 30th Anniversary episode featuring flashbacks starring George Takei as Captain Sulu, but DS9 had a massive, Forrest Gump-style extravaganza that actually inserted DS9 characters into classic scenes from the original series and was absolutely brilliant.
The outbreak of full-scale war between the Federation and Dominion dominated the last two-and-a-half seasons of Deep Space Nine and allowed the show to challenge the ideals of the Federation and the conventions of warfare. Some of this material was controversial with Trek fans but enormously popular with critics and the general audience. Ultimately, DS9 reaffirmed Gene Roddenberry's ideal future for humanity as a worthwhile endeavour, but also stated that maintaining those ideals was going to be incredibly hard. This part of the series also saw Star Trek doing something it never had previously by developing a recurring cast of bad guys - Kai Winn, Gul Dukat, Glinn Damar, Weyoun of the Vorta and the leader of the changelings - and checking in with them as well as the heroes, giving greater depth than is normal to the "enemy".
Deep Space Nine ended in the spring of 1999 with a massive arc spanning nine episodes and wrapping up the storylines of dozens of characters established over seven years. DS9 bears the distinction of being the only Star Trek series to end on such a unanimously-applauded high and leaving the audience wanting more (in comparison, TNG's finale, All Good Things, is excellent but most of its final season was poor). However, there was a feeling that the show had snuck under the radar a little bit too much and its achievements and quality had not been fully appreciated in its own lifetime, not helped by the perceived (but largely illusory) rivalry with Babylon 5, which attracted far more press attention for pretty much just not being Star Trek. Paramount also showed little interest in a DS9 movie after it finished, to the disappointment of fans and the production team.
The cast of DS9 in its final season.
In the seventeen years since DS9 finished, the show has undergone a cultural reappraisal. TV Guide later said that DS9 "is the best acted, written, produced and altogether finest" Star Trek series and it is hard to argue with that. Ronald D. Moore, who worked on three different Star Trek shows, named it his favourite and took many of the writers over from DS9 to work on the rebooted and critically-lauded Battlestar Galactica (2003-09), which was both his development of the ideas from DS9 and his rebuke to the limited vision and ambition of the perenially underachieving Star Trek: Voyager.
Voyager had launchd in January 1995 in a blaze of publicity and ended seven seasons later in 2001, but it is far to say that compared to DS9, it's critical reception was altogether more variable.