Can you expand on what "more robust" means? You know I hear plenty of people wanting the Finder to be better but rarely do I hear suggestions. I honestly don't quite see what is wrong with it exactly other than it not having a particular file hierarchy breakdown on the side. What do people want with the Finder? Putting it in Cocoa won't change its functionality, it's just rewriting its code.
In re-writing the code, they may add some functionality, especially as something more to differentiate Snow Leopard from Leopard.
Let's see...this would be easier to write from Windows but I don't feel like booting up in it.
Checking folder/file sizes with Show Inspector, the Windows file explorer just shows the size of selected items in the corner (as long as you have the Status bar on...that should be on by default...silly Microsoft.) A bit of a time saver.
I prefer using Windows' two-pane view to move files around than any of the OS X views. It would be nice to see that same way of browsing implemented as an option.
Native NTFS Read/Write would be nice (yes, I realise Windows can't read/write to Mac drives out of the box, that would be something nice to see over there too)
Improving the Trash, at least bringing it up to the Windows' Recycle Bin level of functionality.
Adding Cut/Pasting of files.
...all I can think of for now, there are probably a couple other things I can't remember.