Some of the DLC, like After Dark (adds day/night cycle with changing resource use depending on the time of day) and Mass Transit (adds a bunch of new transportation methods along with new roads) feel almost essential to the game. Most of the others (like Parklife, which adds the zoo and some other stuff) just add a little more to do in the game once you've nailed down what it takes to run a city.

And then there's the radio stations, in case you wanted to pay $4 to listen to the same 3 songs and 4 fake ads on loop.