3) DS9 Finally Gave Us Some Action!
Since the Star Trek was conceived at a time when the Cold War was happening, they stuck to a peaceful future, one where humanity had evolved beyond wars. TNG also stuck to the same premise, where even diplomats could go to any extent to avoid wars.
But, all this was still far away from the real action, a reflection of the society we live in. DS9 gave us that with the introduction of the Dominion. The breakdown of the peace talks, and the insistence of the Dominion to rule, all gave us some real-time action, unlike the Star Trek that we had seen thus far.
4) The Huge And Diverse Ensemble Cast
Unlike the other Trek series, where you would see a typical pattern of casting with seven main characters (a Captain, first officer, science officer, engineer, helmsman, doctor, a young one) a kid, a guest star and some fleeting characters. Here was DS9 that broke this pattern and introduced us to the world of many characters, all important in their own way, and the very first Klingon crew member, Worf.
There were nine main characters, and many other thought through characters with small but important roles. So, there were spies, love interests, Dominion operatives, extended families of these people, Generals, and Grand Nagus Zek. There was a whole diversity we could see that we had never seen before on Star Trek.