It's 10" high, which is unexpectedly compact. It seems like the designers have chosen a shape to maximize use of space. I'm sure it isn't the only way to go but it's a wonderful next step.
Again Apple has shown us what we want when we thought we just wanted "more".
But at what cost? Everyone likes to bash big box PCs but the fact is that every single component of one can be swapped out for another. Something tells me that won't be the case for the Mac Pro, and when all it's going to do is sit under my desk, I can afford to have it be a little less compact.
There's the overall computing market, which overwhelmingly favors laptops. A subset of this market desires powerful desktop computers. An even smaller subset desires a self-serviceable desktop computer.
For everyone else, particularly people who who use these machines to make a living and are not overly technical (and have no real reason to be), a Mac Pro with a good support contract attached is far more valuable than one they can open and tinker with.
I think this particular point is overblown. Everywhere I look developers have overwhelmingly switched to MacBooks to little ill effect - machines that are far less expandable and less maintainable than these Mac Pros. It turns out that, if you use these machines to make money, it makes a lot more sense to let Apple take care of problems than to roll up your shirt.
Doubly so if you can't tell the front of a DIMM from the back.
Developers will continue buying MacBook Pros in droves. The Mac Pro is overwhelmingly a media machine.
That custom PC swappable component issue mattered when the only game in town for hermetically sealed boxes was USB (or Firewire).
And even then, this chassis looks like it'd be quite easy to upgrade the internals[0].
[0] http://cdn.macrumors.com/article-new/2013/06/newmacpro.jpg