The Wii U's biggest flaw seems to be that Nintendo failed to take into account the direction the global market has been going toward.
The Wii U isn't a bad device and works well for the Japanese industry where many are suffering with the rise of high cost of HD development and hundred person teams for one game to try to woo the larger Western markets. But it is obvious that they didn't really consult any western developers or take their suggestions seriously in terms of what needed to be done given how outside of the indie scene so few are willing to come to its defense or even bother trying to push the device.
The Wii U is probably the laziest piece of hardware Nintendo put out primarily because it has potential to be better but because of issues like the implementation of online features, the lack of unique Gamepad centric experiences, games and just a overall lack of polish the device is suffering. Had it been released along side hardware of equal power this might not have been as big of a problem but it was dropped early before stronger competitors like Xbox One and PS4, it comes off like a Dreamcast nice but ultimately to be ignored because publishers and consumers have no real motivation to support it.
It is seemingly having decent sales in Japan despite no real major Japanese developer support outside of Nintendo itself but Nintendo has ways to go to improve its situation outside of their home land and have wasted over a year doing nothing to build the Wii U brand. The brand might still be salvageable but the more they wait to do legit work at creating a unique brand for the device instead of trying to make it a next generation Wii successor for families the window gets smaller.
If games like Smash Bros 4 and Bayoneeta 2 don't big market pushes, then the system is finished.







