There are 2 CPU's. Up to 12 cores they say. 4 ram slots is a pain, but still 64GB isnt too bad, its just more expensive to have more RAM then before

The HDD's expandable externally is a deal breaker for me, i hat cable clutter and with that machine there is no way to go around it.
Doubt the GPU's are upgradable, due to apple own thermal design im sure the GPU's have no heatsinks that came with the card, they also look rounded from the images and not as long as the standard GPU's. I guarantee you that they will not be user replaceable.
As they are releasing it with dual GPU's, 10.9 most likely has driver support for that so maybe older Mac Pro's will get those too.
It'll be a beast of a computer, especially in that size, but a pitty everything will have to be external.