Begun while The Next Generation was still on the air and finishing up when Voyager was just starting to really embarrass itself, Deep Space Nine has all the strengths and weaknesses of any middle child.
In creating a "gritty" Star Trek, the creators decided that the happy-go-lucky and friendly attitude of TNG had to go. Since Gene Roddenberry had declared humanity free of conflict by the 24th century, though, they needed less enlightened aliens in the main cast to create interpersonal conflict. If that sounds a little racist, well, it can't be - just look at the captain!
Commander Benjamin Sisko
Barack "Badass" Kirk-Obama, Jr. VII
Okay, he doesn't get promoted to captain until he sprouts a goatee, shaves his head and threatens to cap an Admiral's ass sometime in the third season. Also, he doesn't really want this assignment and is pretty grumpy about it at first. He might appear to be rather humorless, but that's mostly due to Avery Brooks' lack of enthusiasm playing the role.
Sisko is a widower, having lost his wife in the Battle of Wolf 359, when a single Borg vessel crippled Starfleet by destroying 39 starships. This number will become ridiculous by the end of the series, when hundreds of kitbashed models are thrown into battle scenes and blown up with abandon.
Major Kira Nerys
If a black man in charge wasn't enough for you, how about a spunky woman as second in command? A member of the host planet's military, she kind of wishes Starfleet weren't there, but grudgingly acknowledges that they're better than the Cardassians.
After all, they have cake.
Security Chief Odo
Also not a Starfleet officer, Odo isn't even really a humanoid - he's more of a mass of goo that can conveniently take the form of a humanoid to
save money better integrate with the rest of the crew. He served on the station back when the Cardassians controlled it, but who's in charge doesn't really matter to him. As he sees it, "Laws change, but justice is justice."
Science Officer Lieutenant Jadzia Dax
Dax is both incredibly smart and incredibly hot. Although this is of course impossible in humans, Dax is a joined Trill, which means she has a 300-year old slug in her belly that augments her experience, memory and personality. You might remember the Trill from TNG as a race with a bumpy forehead that can't use the transporter beam because it will kill them and whose implanted slug totally takes over their personality rather than merging with the host. You won't be needing that knowledge because pretty much only the slug thing is consistent.
But the spots go all the way down, so we forgive.
Chief Medical Officer Doctor Julian Bashir
Jules here undergoes one of the more drastic character developments in Star Trek history. He starts out as a young, annoying, horny, allegedly Middle Eastern doctor just tickled pink about being out on the frontier. Once it's revealed late in the series that he was genetically engineered, he starts talking like Spock's math teacher and finally gets laid.
Chief Miles O'Brien
This guy - or the actor at least, in an unnamed role we like to assume is O'Brien - appeared all the way back in the first episode of TNG. He mostly pressed buttons on the transporter console there, but here he's Handyman McFixit, with his light-up spanners and whirring woo-woo gadgets that magically repair electronics.
He brings his wife Keiko and their daughter Molly. Keiko is a savvy botanist, but on DS9 she only finds fulfilment by becoming a plucky, stereotype-defying teacher.
Sadly, in the 24th century, replicators have rendered obsolete the ideal profession for O'Brien wives.
Whoopi Goldberg made the role of bartender so essential on TNG that the new show just couldn't go without one. Instead of a wise old crazy-hat lady, Quark is a selfish crazy-ear guy, also known as a Ferengi. His nagging mother ends up schtupping the leader of the planet, his dopey brother becomes the new leader and his nephew joins Starfleet and gets more lines than Sisko's son. Quark keeps the bar and remains named after a subatomic particle.
By the end of the third season, Deep Space Nine (the series, not the station) had garnered disappointing ratings and mixed fan acceptance. Fortunately, TNG had just ended and Michael Dorn was out of a job, so he moved to DS9. Onscreen, the reason was that the Klingons were getting antsy about the folks on the other side of the wormhole, and shoot, Worf's a Klingon, right?
In addition to old foes like Klingons and Romulans, DS9 introduced us to a galaxy of conniving villains and loyal allies. And by "introduced" we mean "riffed on something TNG thought up."
An ancient, deeply spiritual people, Bajorans are distinguishable from humans by their perpetually wrinkled noses and a propensity to blame/credit "the Prophets" for everything that goes wrong/right in their lives. Don't be so quick to judge, though, 'cuz the Prophets actually exist. They just happen to be non-corporeal beings who live in another dimension and don't experience the passage of time.
Kind of like that one Webkinz I bought three years ago and promptly forgot about.
Since Bajor has been occupied by an invading force for the past few decades, the spiritual side of its people has been somewhat overtaken by its terrorist side. In no way is this an allegory.
Even though there are explicit references to Bajoran spaceflight going as far back as 10,000 years ago, their current technology is just shy of the Federation's.
These are the guys who invaded Bajor, raped and pillaged for a generation or two, and then up and left for no apparent reason. You can tell they're bad because they're ugly, their ships are ugly and the space station they left behind is really ugly.
This here's a lady. No shit.
The whole reason for the invasion seems to be that they couldn't stand a green and blue planet so close to their own, so they mined it until it was as brown and dull as everything they were used to. They could have also just been grumpy that their grossly oversized neck/shoulder veins/bones made it impossible to turn their heads. They eventually got their own planet bombed back into the stone age, so that probably cheered them up. Their technology is also not quite as good as the Federation's.
Here we go, a foreign power introduced by this show alone. Native to the Gamma Quadrant on the other side of the wormhole, the Dominion is made up of dozens or hundreds of worlds, but we mostly see three races: the Founders, reclusive shape shifters who hate all us non-shifty "solids"; Vorta, the public face of the empire; and the Jem'Hadar, the bristling military might.
They're all kind of dicks.
The latter two, it turns out, are genetically engineered, and the dino-humanoid Jem'Hadar are kept in line by an addiction to a drug called Ketracel White. Since only the Founders can provide it, there's never any risk of them rebelling or changing sides. The Vorta, on the other hand, are just ass-kissing simps.
Like any truly scary foe the Federation encounters, the Dominion's technology is at first superior, but the difference is soon overcome through research and human ingenuity, because that's What Star Trek Is All About.
Casually mentioned a few times in TNG and still only fleetingly seen here, you'd think the Breen would be a small, fairly unimportant race, right? Apparently they were just waiting for an excuse to fuck shit up, because in just a few episodes they manage a devastating sneak attack on Earth, change the course of an interstellar war and render those vaunted Federation shields nigh useless. They would be so cool if, when we finally get to see one in person, it didn't look exactly like Princess Leia's bounty hunter disguise in Return of the Jedi.