The only thing I don't like is that you seem to have to force it to search for new messages if your 3DS is open and not in sleep mode. It's very slow process. Hopefully they can make it a bit more seamless with updates...
You unlock stuff at a snail's pace in Freaky Forms as well. I don't like this side of Nintendo. It's as if they looked at iOS games and saw the grinding in the freemium-type games, but missed the point.
I'm going to take a wild guess and say this app will be built in to animal crossing 3ds. You will be able to, in game, send messages to any of your friens regardless of them owning animal crossing. You will also be able to read any message sent to you while in game. This will be the mail box feature in the game.
I had a neighbor with one and with his help I got through Find Mii twice, but he seems to have moved or gotten rid of his 3DS. Else than him, only time I've gotten streetpasses is in New York City and Montreal, pretty much.