We all know there has to be a catch for every Nintendo game with an online portion, always. So I guess this is the one for Tri Force Heroes:
Source: http://www.pocketgamer.co.uk/r/3DS/Legend+of+Zelda:+Tri+Force+Heroes/news.asp?c=67661
...You can only play with people on the same region. That should help with latency issues, but is a shame if you wanted to play with friends from around the world.
Also, there's no voice support. When we played, we had to use Skype to communicate. You can send quick emoji-like messages ("over here!" "Totem up" "Item!") which should help out, but if someone doesn't understand the puzzle or how to defeat a boss it will be infuriating.
Source: http://www.pocketgamer.co.uk/r/3DS/Legend+of+Zelda:+Tri+Force+Heroes/news.asp?c=67661