I'm writing this mini "review" of Street Pass Mii Plaza because it's my favourite "game" of all the built in 3DS software and I got my first "genuine" hit on this today. Plus, I haven't seen much other info about this elsewhere on this site.
I suspect Mii Plaza could be easily overlooked in the excitement of owning a 3DS. The icon looks like the Mii channel icon on the Wii and the name of it suggests that it might just be a way of viewing all your Mii characters together in one place (as you do on the Wii), but it's not that at all. It's a game. Actually, it's 2 games. And they are great fun.
You have to have at least one Mii created when you first start this up, but you can use any Mii, it doesn't have to be the primary one that is used for your friend list. You can configure a few personal details (cats or dogs, aspirations, things you like) and a personal greeting. You also set your favourite game. Now you are ready to prepare the two mini-games.
The first one is pretty simple, you are randomly given 1 panel out of 15 from a randomly selected picture. That's it. In the second one, you are the king/queen of the land and are captured and imprisoned. Again, that is it. To play the game, you close your 3DS whilst leaving it switched on (and wireless enabled) and go outside.
As you "meet" other people who have done the same, their Miis are copied into your game and they queue up to greet you. The next time you load up Mii Plaza, they march up to you one by one and you exchange greetings. After the 2nd time of "meeting" the same person, you can set a custom greeting for them, which is nice for me as both my wife and son now have 3DS systems so we can leave each other little textual presents. Any new Miis are added to your plaza and you can view their little public profile cards at any time.
Now the fun begins. You load up the mini-games again - fortunately you don't have to choose to allocate each Mii to a particular game, they can help you out in both. In the puzzle pieces "game" you can choose to take any one of the tiles that they have that you don't have. You don't remove them from their systems, you just get a copy of them and they get to do the same from your collection. Sadly, sometimes you'll have all the same pieces they have, in which case you get nothing.
The Quest game is where the real fun is, though. Basically it's an RPG. The Miis have a number of rooms to go through before they rescue you with tougher enemies (ghosts) to beat in each room. To fight the ghosts, each Mii gets the change to cast a spell or fight. If they cast a spell, they then immediately get exhausted an leave. If they fight, they'll attempt 3 hits and if they don't defeat the ghost they then get scared off.
As with so many things made by Nintendo, this appears to be a simple little mini-game, but it turns out to be a lot deeper than you'd expect. Depending on the colour of the Mii, they get different magical abilities so strategies immediately come in to play. You are better off building up a stock of "heroes" before going in to battle as some magic boosts the power of the heroes waiting to attack, some freezes the ghosts allowing the other heroes to plow through them etc.
After clearing some of the ghosts out you find treasure chests which give you hats that you can put on your Mii so that when they greet other people they can see you showing off your treasures.
I'd expect that the thing going through most peoples minds at this point is "well, this might work in Japan, but where I live there's no chance of ever getting any exchanges". I pretty much expected that, but first of all, there's nothing stopping you getting hits from people you know. I get 2 or 3 a day from my wife and son (I think after a hit there is a cooling off time during which it won't register the same person again) and as mentioned at the start of the article, I got a hit with a stranger today out shopping (funnily enough, my wife didn't get the same hit even though she was right next to me at the time!). Also, in case you need/want help with these, you can use your play coins (2 at a time) to buy puzzle pieces or to hire "adventurers" for the Quest game. If you say you prefer cats, your hired mercenaries are cats...
The last thing I am going to say in my epic post is that this is probably the cutest thing I've ever seen. I don't know what it is about it, but it is incredibly cute. I'm really worried that most people won't even realise this exists with the AR games and Face Raiders to distract them, but this is by far the best piece of built-in software in my opinion. It is your duty to activate these games if you haven't already!
Good review.
) past a lot of people and houses, I've gotten precisely...0 hits 
) I want more people to walk around with their dang 3DS. 
