The iPad Pro, when it inevitably arrives, might be defined by a larger screen, TouchID and other hardware enhancements we may not yet be aware of.
However, it shouldn't be the hardware that defines it, it should be defined by the software, and what the software allows you to do on the new device.
Much as I love my new iPad Air, and am in awe of the tremendous power under the hood, the constraints of the current single application mode of working in iOS stops me from using the iPad as a primary personal computer. The compromises and effort required to do anything but the simplest of tasks, relegates the current iPad to a secondary device.
A very useful one, but for my workflows, I could never use the iPad as my primary device.
This post by Kyle Baxter nails it...
...iOS still hews closely to the one-app-at-a-time, app-as-silo concept that began with the iPhone in 2007. For some purposes, this actually makes the iPad a better tool than the PC; the iPad’s limited ability to multitask makes it easier to focus on writing or other focused tasks like sketching. But it also significantly constrains what’s possible on the device. Writing an email or article that references a website or note, or doing any work that requires looking at one application’s content while working in another, requires flipping back and forth between applications, which makes for an excellent test of patience. And there is virtually no support for allowing two applications to work together on a single task by passing data between them.
Hopefully iOS8 will bring the necessary features required to elevate a future variant of the iPad to a true Pro machine.