The one where we ask "What is the point of Miis on 3DS?"
After a few cursory portable outings on the DS, Miis are a large part of the 3DS from the outset. On the main menu, you have two Mii-related options to choose from: Mii Maker and StreetPass Mii Plaza.
Mii Maker is, for the most part, the Mii Channel from Wii on the 3DS. You can look at your created Miis and make new ones the old-fashioned way with a few updates. You can also take a picture of yourself or a friend and have the system make a Mii from the picture. Sometimes, this works well; however, you usually wind up with some sort of freak Mii that must be taken out back and shot before the world sees the horrors.
You can also export QR codes and pictures of your Mii to the 3DS camera application. So you can share your Miis with the world in a more convenient manner than the Check Mii Out Channel. Sadly, there isn't a way to try to look for Miis of Yoda, Dr. Zoidberg, or Alf on the system yet, though you could awkwardly transfer those Miis over from your Wii to your 3DS if you really wanted to.
The StreetPass Mii Plaza is essentially a beginner's guide to StreetPass. After turning on the StreetPass functionality, you can play two different games. The first, which is boring, is called Photo Swap. You can either use your friends that you pass by or Play Coins, which you earn by using the system as a pedometer, to unlock pieces of a puzzle. The puzzles are all based on Nintendo properties, but sometimes you get a piece you already have. It might be cool to see Link in 3D glory, but it's frustrating to get all the pieces.
The second game, called Find Mii, is Nintendo's version of Farmville. Your Mii, complete with a crown, is trapped in a cage. So, by using friends you meet in StreetPass or warriors cats that you hire with Play Coins, you fight ghosts, collect hats, and save your Mii. On paper, it sounds really simple and dumb, but its stupidly addictive. I'm at a point where I wait with bated breath to get my 10 Play Coins a day so I can burn them all on hiring cats and fighting off ghosts. It's awesome. Also, my Mii can now wear a Red Pikmin hat in Mii Plaza. How cool is that?
Miis can also be used in different games, including Pilotwings Resort, and they serve as your avatar online. Their inclusion is the equivalent of pepperoni on a pizza. Is it necessary? Not at all, but it tastes better with them there, given that you like pepperoni/Miis.