Those single screen mock-ups look really ugly to me. Something about all that wasted space on the 2DS I find more appealing. Seriously.
As has already been pointed out, the 2DS is not a Nintendo tablet. A Nintendo tablet should be horizontal, like a WiiU gamepad controller. This 2DS, as has already been pointed out, has more of a gameboy (advance-ish) vibe. I think it's great, and, along with the WiiU price drop, I think Nintendo has done exactly what it should have done what's more, these perfect moves were unpredicted.