Intel Core M processor, 20nm nVidia Maxwell GPU, PCIe SSD.
This has the potential to be both very powerful in terms of CPU and GPU and yet still be incredibly small and cool.
But since Apple is more about looks than functionality, they won't put a GPU inside the mini and use the built-in "Intel HD" GPU instead. Hopefully they will lower the price by 200$. Imagine a Mac nano for 399$!
edit: I'm looking at my Mac mini 2010 right now, with an external WD 2.5" drive on top of it, and I can imagine a Mac nano being the same size as that external drive.
The Haswell GPU is plenty powerful. It's not much slower than the GeForce 750M