Meeting New Friends.
Playing video games is fun, but there's a lot more fun to be had if you can find a way to play with other people. The husband frequently plays online with his Xbox 360 buddies, but I always had a hard time finding people to play Nintendo DS games with. Some Nintendo DS games tried very hard to connect players with each other - Dragon Quest IX, The Legend of Zelda: Spirit Tracks, and The World Ends with You all allowed you to trade in-game bonuses via Tag Mode - but I never found any other players using Tag Mode in the wild. (I did find a bounty of DQ IX players at PAX, an annual gaming convention.)
A few mobile games (Foursquare, Gowalla) have resolved this temporal communication problem by connecting game data to a location. If a player leaves an item or checks in at an office, restaurant or park, the next player will see that interaction whenever they interact with that location - minutes or even weeks later.
With the Nintendo 3DS, Nintendo has made it a lot easier for folks to meet up. Instead of having to trade data for one specific game, Nintendo 3DS games can opt into the Street Pass service, which will trade data for all games using Street Pass. So instead of only trading data when two players are using the exact same game at the same time, players now only need to both be using a Nintendo 3DS. With the 3DS, I manage to meet some new players every week or so - mostly just random folks walking around downtown Seattle.
To encourage Street Pass use, the Nintendo 3DS even included a pair of Street-Pass focused games with the system. With Find Mii, avatars of the real people that you meet will fight monsters to rescue your avatar, and with Puzzle Swap you can exchange puzzle pieces with folks that you meet. I love these little features which make me feel connected, even passively, with other people who are also playing Nintendo 3DS games. Every day when I come home, I check the 3DS to see if I've made any new Street Pass friends.