Problem is that success is owed primarily to the novelty of the Wiimote and monopolization of the casual market. The Wii U will arguably enjoy neither. Kinect/App Stores have gobbled up a lot of the casual market and the Padlet is hardly on the same level as the Wiimote. Now that is not to say Wii U won't still dominate--maybe the marriage of touch/buttons/sticks will turn out to be a game changer, or maybe the Wii branding will carry it. We don't really know. Point is the success of Wii is evidence of nothing relating to Wii U, it was a very different set of circumstances. Nintendo IP's are a great asset, but they're no guarantee of success--just look at Gamecube.
Actually, there is a novelty in the Pad that is largely overlooked, it is mainly aimed at families who don't have many TVs to spare for videogames. In japan it is reported that many households only have one TV and so, buying a WiiU will have a great competitive advantage that competitors will struggle to match. In addition, assuming these rumors are true (which they probably are), it means the WiiU will be much cheaper than the competition and hence, garner more sales. Finally, even if the WiiU is largely inferior in terms of power to its competition, the architecture will make it easier for developers to port their games onto the system than they had with the Wii, which hopefully translates to more support regardless of the raw numbers.
One thing is true though, WiiU is aimed at a different market than the Wii, but the premises are the same. Think of it as applying the Wii concept to the 'hardcore' market. Who knows how it will pay off though.