FUN! FUN! FUN!
The calendar is filled with cool events, collecting is better than ever, and the interaction with the AnimalCrossing community via websites like Reddit or through the game add a huge new dimension to the game.
Among my favorite features in this particular game is the ability to showcase your home with anyone you connect with via StreetPass, and there are some cool things which you can get via SpotPass (by connecting to SpotPass at McDonalds, Best Buy, etc.)
The internet was in its infancy when the original AC was released. Now, there is a whole world of AnimalCrossing players who love to share and help etc.
Definitely one of the must-have games for the 3ds.