The biggest mistake Nintendo made with WiiU is tying it to the the Wii in terms of branding. I've talked to many people who bought the Wii and dont even realize WiiU is a different console. They just think its a tablet addon for their Wii. 

As the motion gaming and "waggle" associated with it became a cliche over the last few years, Nintendo should have realized that they can't milk the blue ocean anymore with the exact same shtick and that the majority of their Wii owners aren't going to run out and upgrade on day 1. Considering that majority of Wiis have been collecting dust for the last 2 years due to lack of quality software, it would have been wise for Nintendo to leave the Wii image and branding in the past and move on.

WiiU should have been a brand new console in terms of design, with a new name, new image, new marketing message... WiiU is actually a decent console with lots of potential but the mass market has moved on from what made the Wii such a success, and they can't differentiate much between the two.