I had software eons ago that would popup a tree on drag/drop so you didn't have to open the folder first. How many times are you moving things from pane to pane (or lister to lister) and bloody hell, you need to activate the pane, open the folder, then go back to D&D the file/folder. IF it could popup a tree on hover (0.5sec or so) it'd be killer feature!! Hey, I'm the guy who requested the thumbnail zoom on wheel so I'm no ijit 
Peace brothers and sisters