In the Japanese version of Tomodachi Life, the Age-o-matic item worked differently, as it only temporarily turned a child Mii into an adult Mii. It is only permanent when used on child couples or mixed adult and child couples who want to marry, and the effect gets immediately cancelled when they divorce.

In the International versions of the game, this was changed so that the effect becomes permanent when used. Because of this, the Kid-o-matic item was created exclusively for the international versions as a way to reverse the effects of the Age-o-matic. (and allowing adult Miis to become children)
Contributed by Azelf89
When a Mii gets a letter from someone asking them to go to the roof, one of the things that the Mii in disguise says is "Our Princess is in another castle.", a clear reference to Super Mario Bros.
Contributed by aspiradream
As Miis are able to play a Wii U, sounds from the Wii U games Nintendo Land and Game & Wario can be heard.
Contributed by CuriousUserX90
After the controversy involving the inability to have same gender relationships in "Tomodachi Life", Nintendo later responded by saying that if the game were to get a sequel, same-sex relationships will most likely be included.
Contributed by Azelf89
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In the Japanese version of "Tomodachi Life", between 8:00 pm and 12:00 pm, a married couple can sometimes be seen taking a bath together in their bathroom, sometimes with their child. This scene was removed from all localized versions of the game.
Contributed by Azelf89
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In the original Japanese release, the daily donations are collected in a wooden box, whereas in the western versions they're collected in a silver piggy bank.
Contributed by CuriousUserX90
##Nintendo## made a commercial using Tomodachi Life's Concert Hall feature to celebrate the announcement of The Legend of Zelda: Majora's Mask coming to 3DS.
Contributed by Psychospacecow
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In the Japanese release there is an event that can be seen at the Fountain called "Shiritori", in which a category will be chosen at the beginning of the game, such as general word association or food association. Two Miis will then come up with words which begins with the final kana of the previous word, and if either Mii repeats a word that has already been said, they will automatically lose.

The North American version of the game replaces this event with one called "Rap Battle", in which two Miis try to insult each other in rhymes until one of them cannot come up with a rhyme.

The European versions not only contain the "Rap Battle" event, but also an English version of the "Shiritori" event called " Word Chain", where instead of coming up with words that begins with the final kana of the previous word, two Miis have to come up with words that begin with the letter that the previous word ended with.
Contributed by Azelf89
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The Plane (Nintendo 3DS Image Share) and the Observation Tower were created exclusively for the International release of the game, with the Observation Tower serving the role originally used by the Mii Apartments rooftop in the Japanese release.
Contributed by Azelf89
The shop keepers that run the various shops have different appearances depending on the regional version of the game. The Japanese version of the game features shop keepers with Kabuki stage hand (Kuroko) masks, in the American version they have wooden block heads, in the European versions they have robot heads, and in the Korean version they are wearing a yellow racing helmet.
Contributed by Azelf89
The music that plays in the Café area are the songs that Miis can learn in the Japan-only Nintendo DS installment "Tomodachi Collection". In the Japanese version of the game, lyrics (taken directly from "Tomodachi Collection") can be heard during each song. In the international versions of the game, the instrumentals for each song were kept, but lyrics were removed.
Contributed by Azelf89
Before the game was released internationally, many international players thought that a patch in the Japanese version fixed a glitch that had unintentionally made gay marriages possible for Miis, causing fans to complain about this feature being removed, while in actuality, Nintendo was simply fixing a data transfer issue that corrupted save data and prevented people from progressing after they transferred Mii characters from the Japan-only Nintendo DS installment "Tomodachi Collection".

Gay marriages are able to be performed in Tomodachi Life due to the ability to create Miis that have the appearance of Miis of the opposite gender. While many have complained of the inability to have same gender relationships in Tomodachi Life, Nintendo has refrained from including the feature, explaining that the game is not to be looked at as though it is a "simulation" of real life.
Contributed by Azelf89
In the Japanese version of the game, there are only 6 genres of music that Miis can sing: Rock & Roll, Pop, Enka (Japanese traditional song), Opera, Heavy Metal and Rap. The International versions of the game removed the Enka genre while and adding 3 more: Ballad, Techno and Musical.
Contributed by Azelf89