Currently, you need a free space to move items from place to place in your inventory. I would like the ability to have the two items simply swap locations. This would make sorting items in a bag or between bags much simpler.
+1, although I'd be fine with just being able to swap bags themselves, as I usually have space inside bags, but not free top-level inventory slots. The interface for this could just be dragging one bag/container onto another, which currently does nothing.