Everyone makes fun of the complainers, and there will always be complainers, but perhaps some of them are justified.
1. GPU
First, kudos to Apple for actually taking GPU performance seriously this time. However, big question #1: are these user replaceable/upgradable? What type of form factor are we looking at? For some the use of ATI vs. nVidia is non-negotiable.
2. CPU
Big question #2: is this a single socket workstation? If so, if offers only 12 cores/24 threads (at best). Why limit your flagship workstation in this way? We have had 12 cores already for years now. Why not introduce a dual socket model with 24 cores/48 threads. Not everyone needs one, but I'm sure there are plenty of video professionals out there who could use the rendering power.
3. Expandability
Why have all the extra cords to attach ESSENTIAL things like extra storage? No one can afford all SSD-storage anyways -- at this stage in the game HDDs are still absolutely necessary, especially for those storing/backing up HD video. Why make this external? You lose all of the "footprint" and "noise" advantage the new Mac Pro supposedly brings.
As someone said, "If a car could omit seats for passengers, Im sure they would get a lot smaller and more efficient, too. Until the trailer for carrying passengers was attached."
Additionally, how will folks who actually NEED PCIe expandability fair? Good luck attaching that fibre channel card. Even if it is possible via TB, now instead of one SIMPLE box, you've got a million cords attaching this mess of a workstation together.
4. RAM slots
Maybe most people don't need more than 4 slots/64GB of memory, but many actually do. The only way to get this is if a dual socket version is offered (unless you buy 32BG modules, which are prohibitively EXPENSIVE).
Conclusion:
Don't get me wrong, this is an incredible piece of engineering for what it is. The cooling system is fantastic, its a beautiful piece of machinery, and it will be nice to finally have up-to-date CPU and GPU. But why is Apple so obssessed with miniaturizing the form factor of a Desktop workstation, and scattering out essential functions all over the place with expansion boxes? It also seems likely (but unconfirmed) that due to its size, it will not be a dual socket workstation, which limits its compute power and the amount of RAM in it.
Listen, Apple, its cool that you designed a great computer. But you didn't really need to. All most Mac Pro users needed was a big box with USB3 and Thunderbolt and the new CPUs. This did not require a new design that compromises the basics like installing even a single hard drive, and limiting the memory slots. You don't get that boxes connected with cables is inelegant, more expensive, and depends on existence of products that are very limited in selection (e.g. Thunderbolt choices and cost).