So it'll be a big expensive touch screen Wacom tablet?
Pretty much. Tablets aren't good for everything, but for the things they are good at, they're very good at.
For instance, I wouldn't want to do a spreadsheet on one, but for photo editing, drafting, modelling, or really any type of visual design, they're beyond excellent. The form factor is perfect for it. All the iPad needs is more ram and proper stylus support. Both of which an iPad Pro can (and better) provide.
I see where you're going, but even in that representation it'd be limited. I like the idea of using it as a controller / UI extension, but outside of that it can't just be the traditional iPad.
I think that, at least starting out, it's not gonna be that much different of an experience. Unless Apple has some huge changes in store for iOS8, you're gonna be single apping everything, same as you do on the current iPads.
And no, it won't be quite as good at everything the iPad is due to size alone. Games in particular will suffer a bit. Angry Birds might be alright, but anything that uses virtual analog sticks will suck due to the fact that the size will make it too unwieldy to hold.
...but the larger screen and inclusion of a stylus will be what ultimately justifies it. It's a "pro" machine, designed to be a better fit for higher end tasks. The comparative weaknesses will be offset by its relative strengths. Like if you want to play games and surf the web, you've the Mini. The happy medium? The Air. Content creation? The Pro.
Though I will agree with you on one thing, for the iPad Pro to truly succeed in the long term, we'll have to see more functionality added to iOS. What it needs is the UI paradigms of iOS, and the flexibility and multitasking abilities of OSX. Being a supplementary device will only carry it so far for so long.