The Dominion! The Maquis! The Mirror Universe! The Circle! Kor, Kang, and Koloth! Lots more Dukat and Garak! The DS9 Rewatch does a second-season overview.
Having said all that, the season is far greater than the sum of its parts. We get so much in this season, from the deepening backstory on the Cardassian occupation of Bajor in “The Homecoming,” “Cardassians,” “Necessary Evil,” and “The Wire”; to the gradual introduction of the Dominion, culminating in a superb climactic season finale in “The Jem’Hadar”; to the reintroduction of the Mirror Universe in “Crossover”; to the fleshing out of Dax, Bashir, Garak, Dukat, and Rom; to the establishment of the Maquis as a recurring threat. All these things are the foundation on which the next five seasons will be constructed.