One thing a lot of people seem to miss when it comes to how consoles did in the last generation was the massive growth in multi-platform ownership; and, while the PS3+Wii or XBox 360+Wii configurations were (probably) the most common, people have shown a willingness to purchase additional platforms when they demonstrate value.
With this in mind, the challenge for Sony, Nintendo and Microsoft in the new generation is not how to convince consumers to buy their console instead of their competition as much as it is to produce enough value to convince gamers to buy their system regardless of what other systems they already own.
This may (potentially) be the problem that Nintendo is having with the Wii U because, while it is a very fun system to play, I'm not sure Nintendo has released games that really demonstrate that the Wii U is different enough from existing platforms. Nintendoland is full of promise, and many of the mini-games could be expanded into full games that demonstrate this, but (being that they're just mini-games) they're limited in how good of a demonstration they are; and it is difficult to argue that New Super Mario U is effective at demonstrating the new functionality of the Wii U tablet.
Obviously, Nintendo has time to deliver on this, but if they can successfuly demonstrate value there is little reason to believe that they're limited in the sales they can achieve.