If you get a Mii from another person's 3DS you can view them in Mii Plaza and use them in Puzzle Quest and Find Mii.
Puzzle Quest is basically the trading of puzzle pieces to complete pictures of Nintendo games.
Find Mii is a mini RPG where you use the Miis to fight ghosts and break your Mii out of a dungeon. You can also collect hats from the ghosts that your Mii can wear.
Coins are gathered from walking (100 steps = 1 coin but only 10 a day can be earned). You can use them in the AR Games to buy new games, in the Puzzle Quest to buy pieces and in Find Mii to hire heroes to fight for you.
They can also be used in games to buy stuff, like in SSF4 to buy figurines or Lego Star Wars to buy blocks and costumes.
With Street Fighter's StreetPass mode, you can line up a team of figurines that will battle another person's team and you will get points and more figurines depending on if you win or not.
There's also the 3DS Sound StreetPass that shows the top 5 songs that you pick.
Up to 12 programs can use StreetPass at any one time, but it's likely that SSF4 isn't on in a lot of 3DS units yet, because they might not know to activate it.