4. Form factor: In case you didn't catch the article, it uses a 2U server chassis. Holy crap that's big -- at least compared to a desktop. Most of the space within a typical desktop case is occupied by a large 5.25" DVD drive, a large power supply, many large hard drives, a large graphics card, and a little bitty tiny CPU on a tiny motherboard in the back of the case. The 48, 64, 96 core servers are almost solid silicon with CPUs; they don't have these big bulky components. The power supply in those cases is surprisingly small considering how much power it draws. Miniaturization is expensive, and people aren't going to want to carry around an 80 pound laptop or have a desktop with a motherboard as big as one in a 2U (this increases the height and depth of the case and requires at least as much depth as a full ATX, if you use a desktop shaped chassis).
Just a point of correction, a 2U server chassis is pretty small, only 3.5 inches high. A normal full atx desktop case is the equivalent of a 4U rackmount as I recall.