Desktop Reversal
Ever since Windows 95, I've hated the concept of the desktop "folder". This was partly due to flaws in Microsoft's implementation, but mostly because the whole concept seemed broken to me. The desktop is too awkward to access (because you have to minimize other windows instead of simply switching to another window) and lacks a unifying purpose. Mac OS X Panther softened my resistance somewhat - Expose solves the access problem, and on the Mac it's generally easier to keep a clean desktop (As a rule I don't use the desktop on my powerbook for long-term storage of anything. Right now my desktop is completely blank).
Now enter Tiger and Dashboard - a hidden "layer" of widgets that can be brought to the forefront via a hotkey or an icon in the dock. This works the way I always wished the desktop worked in Windows. But now, I find myself wishing that the Dashboard widgets showed up on the desktop, so I could see them under my active windows and didn't need a third hotkey (after mapping the bottom-row enter key to expose all windows and command-enter to expose desktop). It's almost there - you can drag widgets from the Dashboard layer to your main screen, but they always display on top of other windows. Hopefully some enterprising hacker will figure out how to turn off the "always on top" bit (an "always on bottom" bit would be nice, but not necessary).