A new feature in the Mii Plaza tracks what you have and have not completed
http://www.nintendoworldreport.com/news/28616
Upon installing the recent update to the Nintendo 3DS and visiting the Mii Plaza, players will be greeted by a bounty of new content to enjoy. Accomplishments, a new feature that tracks a players achievements is now available.
In the main selection screen of the Mii Plaza players can now select the Accomplishments category. Within this menu is a list of various different tasks players can complete. These vary from completing a puzzle swap panel to simply filling up your Mii Plaza's population. There are a total of 78 accomplishments, some bearing unknown requirements. These unknown accomplishments simply show a "???."
Some of those achievements look to be impossible for the average person to get (for example, how would you get 100 StreetPass encounters in 1 day unless you go to something like E3 or PAX?).Live in a Japanese city.