This is possibly a bit old, but I've not seen it mentioned, so here we go. I'll sum it up.
http://www.n-sider.com/articleview.php?articleid=472
Summary:
- You travel into the game in a car, and travel around in said car.
- The train station is gone, it's now a 'main gate', guarded by the police dogs from the original Animal Crossing (Booker and Copper). Talking to Copper (the one that wanders outside the police station in the original) lets you go to other towns (either via local wireless connection or WiFi), check for lost items, change the flag or get some kind of hint.
- To visit other peoples' towns over WiFi you have to know the name of the destination town. To avoid strangers appearing in random towns, players create a friends list. Friends can be added manually or by clicking an option when using wireless local mode. You can visit the town of anybody on your friends list providing that they are online and their copy of the game uses the same alphabet.
- Real life holidays are now gone and are replaced with Animal Crossing specific ones, to make the game more compatible for online play.
- You cannot leave your town without making sure all guests are gone. You cannot get into towns that do not have their owner in them, and they cannot get into yours while you are offline or visiting another town.
- You can control your character with either the D-Pad or the stylus.
- The top screen is used for the sky to show weather effects, the mail pelican, balloons, stars (which can be connected into your own constellations), etc.
- Players can alter the game time without altering the DS system time. This means you can play the game's days when it is night in real life, for example.
- The menu is split into distinct groups: briefcase (holds up to 15 items and 10 letters and shows your money, you can customise your character's appearance here), drawing board (design your own cap, home, art, shirt, umbrella, rug, or wallpaper), fish table (fish you've caught), insect table (insects you've caught), communication box (communicate with others in real time - a chat program), map (can show town buildings and houses seperately) and escape (goes back to the main game screen).
- Tom Nook's shop may now be on two floors, with a salon upstairs. A point system used to get discounts will also be in place - get 300 points (amassed with multiple purchases) and you become a 'member' and get further discounts.
- The post office and town hall may now be integrated into one.
- The largest house in the demo played had two floors, but it's said to feature much larger houses.
- Multiple characters can enter a single house at the same time.
- Players can now apparently choose where they want their house in the town.
- Four players can play using the same card, living in the same house. As you make your house bigger, you can give players seperate rooms.
- Full camera control is available inside your house.
- A telephone is in the house - perhaps this is used to 'call' other DS' or for communication with the other inhabitants of your town.
- Storage will be easier - fridges, cupboards or other furniture that can store items can hold 90 items (stored in 6 different compartments, 15 items in each).
- A feature where somebody can draw something in Pictochat then send it directly to a bulletin board in somebody's town is being considered.
- Animal Island is most likely omitted, and NES games are definitely out.
- There may be a character who appears that requires that the player draws them a face, such as in the first game. However, this one will be saved and may randomly visit others' (off your friends list?) towns telling them about you.
- Players can interact with each other better than they would with non-playable characters and can kick sand at each other, and perhaps host fishing tournaments, play tag, etc.
Phew. Hope you enjoyed all that. This game is looking so great. I'll be buying it on launch day, but whether that's American launch day or UK launch day remains to be seen.