14mm is a pretty nice thickness. The Vita-1000 was one whole part instead of a clamshell design like 3DS, and was way too thick and heavy for its size (18mm). Vita-2000 slimmed it down to 15mm, and it's quite a comfortable thickness. So 14mm for the dev kit should make for a portable unit that's nice to hold, and for detachable controllers which feel quite nice in the hand to grip.
I still don't like the idea that it's actively cooled, though. No portable device shipping in 2016 should be, though if it means unthrottled performance on the X1 in a tiny form factor I guess it's worth it.
Still, it's a shame Apple makes the best SoCs on the market. Last year's A9X and its twin Twister CPU cores run rings around the X1, and its GPU is basically a match. And it doesn't need active cooling, doesn't throttle (though the GPU was slightly underclocked in the 9.7" iPad Pro) and fits inside a thin form factor like the iPad Pro 9.7". But it's also 16nm compared with X1's 20nm, so I wonder if Nvidia has something more modern for Nintendo by the time the system launches.