From what we saw at the Nintendo Media Summit, the city is just another place to go. Instead of Tom Nook's shop in your own town, you can buy from an upscale store. Instead of waiting for the fortune teller to visit, you can see her in the city. A few minor additions include a city-based auction house and theater where you see shows.
The rest of the updates seem equally subtle. You can play with your Mii face. Up to four players can meet in one of their towns or the city. Gamer can talk over the WiiSpeak microphone (sold separately). You can type messages by plugging in any USB HID keyboard. Otherwise, it's Animal Crossing again.