There's little doubt in my mind that Wii U will excell by far the most in Japan (relative to population difference of course). They tend to favor the handheld/tablet style gaming which the Wii U somewhat represents, not to mention its quirky Japanese style games like Nintendoland. It's funny, because I actually thought this would be the case with the Wii, but it turned out to be the opposite.
However, if these second week numbers for the US are any indication, this is pretty troubling for the console as a whole. Japan can only do so much, and they are not as significant in the gaming world as they once were. Look at MS, they've managed to outsell the NES with virtually no success in Japan. In other words, it's a far better scenario to have success in most other markets and not Japan, than to ONLY have major success in Japan, which I fear the Wii U is getting set for. And unlike the 3DS, that can get away with these sales because of its lack of significant competition, the Wii U WILL have strong competition, even if it comes a year later.
Barring a sales miracle that rockets it up like a Wii Fit type, or Monster Hunter for PSP, I see the Wii U as possibly the next N64 in terms of sales, and stuck in neutral for most of its life. Perhaps average sales, but a major step down from the previous gen, and possibly outsold by competitors.
