Nintendo finally started dipping into paid downloadable content recently with New Super Mario Bros. 2, as well as Fire Emblem: Awakening in Japan, but that doesn't mean the company finds it suitable for every game. Animal Crossing: New Leaf is one title in particular that won't be receiving paid DLC, as Nintendo President Satoru Iwata says it's not wholesome enough to fit in with the Animal Crossing philosophy.
A financial Q&A translated by Kotaku quotes Iwata on the matter: "That sort of element is absolutely not being added." According to Iwata, monetization of DLC would affect the way the game is played, which is interesting considering how much opportunity there is for paid DLC in the Animal Crossing structure. I'd imagine most other companies would leap at the opportunity to charge for the myriad goodies available to collect.
That doesn't completely rule out DLC—Animal Crossing: City Folk had free content to download—but it does mean you won't be paying for it. Hooray? Yeah, hooray.