I think a fairly straightforward but highly productive enhancement would be a Vista-like feature we see also in utilities like "Folder Pilot" (shareware).
Right now we have hyperlinks on the title bars of our windows and we can back up the tree by clicking on them. But what if you could drop a menu from that point with nested menus and navigate through the subtrees to go from anywhere in a branch to anywhere in any other branch, with two clicks total
I just tried this out in Vista after you mentioned it and I agree that it's a very nice feature. In fact, using that feature seems to be a key to making the Vista File Explorer usable - I was kind of finding it clunky until I started playing around with that way to navigate...