One of the nice things about the Mac Pro was that it was engineered to look good, as opposed to being "designed" to look good. (or not depending on your design tastes).
The modern PC has three things you have to manage when engineering a 'new' one, heat management from both the CPUs and the GPUs, signals management which can carry exceptionally high frequency signals across "busses" that act as transmission lines, and modularity.
This vision has the latter covered but introduces challenges that make keeping the other two in check probably impossible.
First (and perhaps foremost) a modern PC generates a lot of heat, and managing that heat is central to any design, like having a place to eat and a place to sleep are essential to any home design. The components will generate up to 1200W of heat (equivalent to a space heater or hair blow-dryer set on 'high') Which needs to be removed from the components fast enough to prevent them from reaching their critical temperature.
Next up, modern PCs use PCIe channels that clock at 5Ghz with low voltage differential signaling. These signals absolutely do not like going through connectors, so systems design need to minimize connector transitions to 1 or zero. The renders for this design seems to show components plugging into a back plane (minimum two connectors). Also at 5Ghz, the wavelength of these signals is 6 cm. That means that bits going through a wire that is longer than 6cm long start taking a "long time" (relative to the signal) to arrive. Now PCI has some mechanisms in the protocol to support this but it starts resulting in additional bus transactions and signals. That increases latency in a way that can turn your awesome GPU into something decidedly unawesome in terms of its performance. It isn't the GPUs fault, its getting the data too and from the CPU which is the problem. There is a reason the 'video' slot on PCs is the one right next to the processor :-). This problem is made even more manifest in memory systems. If you get a chance to look at the circuit board traces for memory DIMMs some time notice how some are really squiggly and some are straight. The trick is that they are all exactly the same length so that data that starts coming out of the chip arrives at the same time at the CPU and vice versa. Fully Buffered DIMMs (FBDIMMs) were a shot at 'fixing' this and few people bought them. The chips ran hotter and performance was worse, lose lose to most consumers.
All that said, I love the concept of modularity in a PC but at the moment I can't see how to get that without sacrificing a lot of performance. That is one of the reasons I was so impressed with Apple's effort. It gets a lot of things right.