Stacks in the Dock on Leopard are great - incredibly useful and easy to use. But subfolders in stacks revert back to the Finder which I find quite frustrating.
If Apple wanted to be even more cunning, I have two idle thoughts...
First: I would be able to click or hover over a subfolder and see it's contents - but stay in the Stack view of things. The modal change is too quick at the moment - it's like Stacks were only half implemented and you have to do the rest with Finder.
The second problem is that - whilst they're great for finding stuff and dragging stuff around - filing a copy back to Stack from a Save Dialogue is just as slow as it has always been. I often use Stacks to access deeply-buried-folders - so they can stay sitting in their rightful place - but readily available as a Stack for quick and frequent access.
When I use an application and I want to, say, save a new copy back to the location of the Stack I still need to navigate to the location of my deeply-buried-folder. What would be ideal would be a "Stacks" group in Finder (and File Open and Save dialogues) - which always shows me which Stacks I have on the Dock. So then I'd have quick access for both retrieving and creating.