A real long-term motivation you actually do not. Not more than the typical "Come back or your city suffers" method.
You get a typical Animal Crossing. Anyone who has played the predecessors, is noted that the basic game has not changed, except that it shines in rich 3D and uses the various functions of the 3DS.
Per StreetPass you can get homes of other players in a model village and to view or to open his city for a local or Internet game. Here was but said that it is very annoying, if the connection is not stable. As long as you're online, you have to live with the constant fear that his score is reset due to a disconnection, so that all changes are undone. This can be very annoying, but since Let's go to the City a problem.