Will developers be able to fully utilize the screen on the WiiU controller as an integral part of gameplay? Anything that they have mapped or displayed on that controller screen goes away as soon as you send the image that would be displayed onto your tv to it. To me, that says that developers will not be able to put anything on that screen that is crucial to gameplay.
The fact that you can use the screen on the controller as your main game display seems to be a major selling point for Nintendo (it's the first thing they showed in their E3 announcement trailer), so, will they allow games to be published on WiiU that require the use of both your television and the controller screen at the same time? If not, developers may use the controller screen in a way that enhances your gameplay experience, but not in a way that is a necessary part of their game. It's kind of like how the 3D in 3DS games is a cool addition, but since you can turn the 3D off, the 3D can never be used as a necessary aspect of the gameplay.