I need to change my vote to both. But mostly execution.
I don't think it is a great concept; I think it has very limited appeal to people 35ish and younger. (Which granted is most of gaming.) Older eyes just can't switch back an forth, and I think that was a big part of their Wii Fit and Casual market.
But the small screen, poor implementation, single screens (so far), refusal to upscale Wii games, and no or few 'must have' games. It just seems like a Wii+ and should have been released in 2010 at the latest.
Plus I absolutely hate the name. I despise the trend of inventing fake word that aren't needed in any way. Especially ones that sound like other ones. Before I got one I never called a wii a 'Wee.' I called it a 'double you, eye, eye.' I still do that much of the time. They just made it worse by slapping a capital U at the end.
They call it a WiiU, cause you take one look at it and U turn around.