Being a G4 Cube owner and a computer enthusiast for decades, I envisioned a cylindrical Mac 10+ years ago when I realised we are never going to get a decent PC case and cooling solution from the lame PC case market. Windows fanboys were making fun of Apple Cube, iMac G4, PowerMac/MacPro while at the same time they were buying ugly LianLi and CoolerMaster cases (or even worse). Most modders were just interested in silly cathode lighting, ugly windows and overclocking rather than elegance, compactness and quietness, too.
I remember a cylindrical mac concept in an iCreate magazine years ago and I wondered why Apple didn't do it. Apple Cube was a missed an opportunity; it was too expensive, too early, too unexpandable to replace a PowerMac. It did wonders later in the 2nd hand market, being an advanced MacOSX capable desktop that was more flexible than an iMac, more quiet than a PowerMac, classier than anything else for a HTPC. While I would like to see the new MacPro as a product between the old model and the iMac, I really appreciate Apple for putting effort into building something fresh and I hope they still believe there is a future for the desktop PC.
Personally, I don't care about PCIe, internal HDDs, optical discs as much as I don't care about California, but I understand that many people do. Nevertheless, you couldn't replace CPUs in the old MacPros, GFX cards were few and ridiculously expensive unless you started flashing PC cards, the trend for a/v equipment is going external with firewire/usb/thunderbolt peripherals, internal RAID for data might not be as safe as external solutions. I would take OpenCL over CUDA for the future anyday. But I am going to miss the 8 RAM slots. I hope SSD is user replaceable but it will be expensive. Anyway, it would be cool to have an option for a 24core "old" MacPro with thunderbolt, or even better, a 64core (hello AMD), 256GB RAM beast ;-)