I purchased a Mac Pro back in 2007. I pretty much maxed it out at the time with dual quad-core Xeons, 4 SATA 750GB drives and the Apple RAID card. Editing video (FCP), audio (Adobe Audition from CS5.5), and of course RAW photos (Adobe Camera Raw) from my 5DIII, what is shocking to me is how fast, reliable, and robust this machine has turned out to be, even to this day. Yeah, at the time of purchase, I was planning on doing video card upgrades and other expansion over the years, but I really haven't done it. It hasn't been that necessary.
Sure, I dropped upwards of $8K on this machine at the time, but it has lasted me 6 years and continues to perform quite admirably. It has never needed repair, and it's running all the time. I like that!
So, I am not that concerned about the new Mac Pro and expandability. If I decide to pillage my bank account for an upgrade (maybe after Canon releases a high megapixel SLR!), I will just try to max out everything I can afford, then leave the machine plugging away for hopefully another 6-7 years. I do require a CD/DVD drive for media creation, but otherwise, it's hard to complain with credibly about the specifications and myriad port flexibility.
I am by no means purely in the Apple camp having run an IT department for over a decade with mixed Windows flavors and Macs. I see (and hear from users!) both sides. What we've noticed is that even to this day, even our fastest Windows units still benefit from a Symantec Ghost re-imaging of the hard drive every 18 months or so. Maybe this isn't as necessary with Windows 8, but it is with previous flavors. We have not had this kind of issue with the MacOS, particularly the second half releases of Snow Leopard and Mountain Lion. Anyway, the Win/Mac thing is a tired old argument, and few in one camp are ever convinced by folks in the other camp. Personally, I don't care. But I think Apple has the edge at the moment with OS technology, performance, and reliability. If Mavericks is stable, the new Mac Pro is pretty compelling.
One other item on the new Mac Pro: Someone in the thread mentioned rack-mountability (or lack thereof). Funny, I thought of that the moment I saw the black cylinder form factor. I bet right now there are companies out there figuring out ways to get this into a rack. First one to market will make a bundle!