Well, you asked...
The positives: overall power, and Apple's willingness to design something very different.
But...
Design: form over function?
Thunderbolt: slower than the newest PCIe specs.
No expansion: for its sleek industrial design, and small footprint, one has to add lots of external boxes/enclosures for multiple hard drives, PCIe cards, external thunderbolt devices, etc., etc., etc., resulting in a much larger overall total footprint than one has with a standard computer case.
Everything soldered in, cannot be upgraded, easily repaired or fixed.
The big unknown: cost.
Conclusion: probably great for the video crowd, with its dual graphics cards, where expansion isn't necessarily as critical, but not that great for intense audio production, particularly with large sample libraries (i.e. VSL, and others), where expansion matters. Because it is Apple, it will be declared fantasmicalwondergreat.
Conclusion #2 (personal): not a MacPro, but a souped up, and probably very expensive, iMac. It will go on my iGnore list.