One feature I like about the 3DS is the Friend List. It shows you which of your friends is online and which game they're playing. But Mario Kart 7 is the only game I've seen where you can join your friend online by clicking "Join" from the Friend List. All the other games I've seen usually just tell you the name of the game they're playing.
Why didn't they put the Join feature in more games? It would make sense if Mario Kart 7 just came out, but it's been out for 2 years I think. It would be really helpful in Animal Crossing: New Leaf, to know when your friends' gates are open or not; also in Mario Tennis if your friend is searching for an opponent.