I've been Street Pass-ing it up this week with my Nintendo 3DS. You'll see in the above video that I've just hit 70 Miis in my Mii Plaza. Hell yes. While I haven't fared as well in Street Pass Quest, I've done quite well in Street Pass Puzzle. You'll see that I've unlocked several new panels and have actually completed a couple of them.
You complete these panels by running into people on the street that have the pieces you need. You both trade pieces through Street Pass, and later you pick which of their available pieces you'd like from them.
In the above video you'll see what happens when you complete a panel by meeting enough people. It's pretty cool, though it looks way better in 3D. Stick around for the end of the video to see me unlock a couple of new panels.