
There are seven advisors you can talk to, some of which are quite amusing (or annoying). Plenty of land to build, which varies depending on tile size (there are three available, the largest being 16x16 kilometers, four times larger than the sole option in its successor). Huge variety of bridge designs, which gets even better if one installs NAM. Realistic building designs, especially in the high-density RCI (residential, commercial, industrial) section. The largest SimCity 4 community forum is Simtropolis, which has been online since 2002! Some good users include SimGoober, Equinox, mattb325, scotty222, Diego Del Llano, and Jasoncw, among others. Furthermore, there is a massive repertoire of buildings available to download, such as skyscrapers, civic buildings, parks, houses, and shops that aren't in the vanilla game.
Not only did Maxis release official modding tools such as the Building Architect Tool (BAT) and the Lot Editor, but there is a huge library of community mods such as the Network Addon Mod (NAM), the Colossus Addon Mod (CAM), and the Day and Nite Mod (which darkens the night sky as opposed to the default version which has a slight bluish tint).
The modding capabilities are endless, even for 2003.