Deep Space Nine was a Science Fiction onyesha that didn't rely on Science Fiction to solve it's problems. It recognized that actions have sometimes long reaching consequences. People had to make terrible choices without the benefit of a last dakika techno-babble whiz bang magical solution that left everyone imba "Kumbia." "Children of Time" and "In the Pale Moonlight" illustrate this point.
Perhaps it was the static nature of the setting. Most of us ST mashabiki were used to TOS and TNG where each week represented a new adventure. From week to week, we never got to see the fallout from what the Enterprise crew had encountered the week before. DS9 gave us the opportunity to see that fallout, and it often wasn't pretty. It was called the "Dark ST" for a reason.
The other thing that made DS9 different and superior to it's siblings was that the writers dared to give us three dimentional characters who were deep and rich and complex. We saw the real-life scenario of these characters interacting with each other on a personal level. Friendships developed, romances blosommed, comarades had disagreements and fallings out. Is this not a mirror that reflects life as we know it?
DS9 had it's share of action and adventure to appeal to the traditional SciFi fan, but also brought a depth and maturity that distinguished it from other ST offerings. It was and remains superior televisheni and stands up supubly after all these years.
DS9 dared to be darker than the other incarnations of nyota Trek. There was a richer and zaidi ongoing storyline with the Dominion War, which also set the scene for some of the best actions sequences from any of the other series...
It was darker, ballsier, and edgier than the rest and was also the first to not take place on actual ship. Plus it had a black Captain and a female first officer another first for nyota Trek. It's my inayopendelewa of them all.