With a typical console-type controller, MMO's do not work well. With the 3DS's touch-screen, an MMO can work great. Let's take a specific example of how an MMO can work with the 3DS: Wizard 101. Now, if you've played this game (it is free to play), the game-mechanics are arranged so that in battles, you basically just choose between 6 cards. Each card has a different spell or ability. It is turn-based, and each turn, you have 30 seconds to choose a card. With the touch-screen, all 6 cards could easily fit and be easily accessed. The rest of the game controls could be done with the other parts of the 3ds (d-pad, thumb-pad, r/l/abxy). Even the gyroscope could play a role. With the viability of MMO's, through the utilization of the 3DS controls and touch-screen, Nintendo could make their own MMO and possibly bring in numbers higher than even WoW.

Making an MMO for the 3DS should be easy for Nintendo. They could just take their various game worlds (like the Mario and Zelda worlds), and make those the areas of the game. The different classes and races could be done in similar fashion (like, you could pick a mario-esque type of class/race, or a zelda-based type). The quality of graphics on the 3ds, plus the 3D, and the easily setup internet component (in particular, with 4g), enable Nintendo to create a great portable MMO.

On the business side, the value of a Nintendo 3DS MMO is multiple times higher than the value of a regular game. They initially sell the cartridges, then they also charge a monthly fee, and last, if the game is really that great, it would also increase the amount of 3ds sales.

Environmentally, it would also be helpful. I know this sounds strange, but if people would rather play their 3DS on the way to and from work/school, they would be more likely to use carpools and public transportation.

The future potential for portable MMO's, especially 3DS MMO's (owing particularly to the 3D, and the touch-screen), is promising. 4G is still in its initial roll-out stages, but is available in most metropolitan areas (at least, in the U.S.). If I remember correctly, they estimated over 100 million American's would have access to 4g LTE upon its initial release. Of course, they have already expanded and are in the process of expanding coverage to more areas and more people. It's a good market that will keep getting better.

I'm hoping Nintendo does make a 3DS MMO. I'd like to see them defeat Microsoft and Sony, two mega-corporations with their business hands in many different areas; as opposed to Nintendo, who only do video-games. It's difficult for Nintendo to compete, when they have so much less money to work with. A sort of cash-cow like an MMO might be exactly what they need.