"Easy to create an own Look and Feel"
In wpf, controls are lookless. What's that mean? It means they can be styled to look any way you want. So if you want round buttons then you can make them round. You can also style theme and re-skin an application relatively easily. Whack a style targetting all buttons into app.xaml and all buttons will now default to that definition.
Plus, the way binding and commands etc works it's easy to separate concerns. So you can put together a form and viewmodel which are independent of one another. A bit of training on Expression Blend and your graphics guy can completely alter the look of your form. What's more everything can still work without the graphics guy learning to code.
"One thing that developers seem to overlook is that Windows Forms is built upon Windows more directly than WPF"
I don't follow that.
DirectX is part of windows.
With win8 it sounds like the fundamentals of the win32 api are being replaced.
I'm not entirely sure what that means for winforms in the long term but if excel still works on win 8 then I would think most winforms which don't have any p/invoke in them will still work.