I've played Animal Crossing since the Gamecube version, have bought every game and will be purchasing New Leaf. I believe what's kept me playing is the lack of a goal and/or end.
The main attraction for me is the homely (is that the right word?) sense it gives. I enjoy late night fishing, and chatting with the townsfolk, even listening to K.K. slider on Saturday nights. I guess it's more of a time-waster than anything. Multi-player has also played a large role in my interest. Seeing the difference in style of my friends' towns is always enjoyable. Also trading patterns, etc.
I'm so excited for New Leaf especially for the return of portability! Although City Folk and the Gamecube version were great, they lacked what Wild World had: ability to take anywhere! Now instead of having to be grounded to staying at home to play, I'll be able to take my town anywhere and everywhere!
Also, if anybody is interested in visiting each other's towns once the game is released, shoot me a pm! I'd love to gain a few AC friends! ♥