A new feature added to Oxygen for KDE SC 4.5 is the possibility to move windows around by clicking (with left mouse button) in any of its 'empty' area and dragging it around, in a way that is similar to what one usually does with the window decoration titlebar.
How this works is illustrated in the following screencast (sorry for the poor quality of the video):
This feature has been present in bespin (another kde4 widget style) for quite a while. It has been ported (and largely rewritten) to Oxygen, on popular demand.
It is believed to be very useful, notably on touch-screens, for which dragging windows from the sometimes tiny decoration titlebar might end up being problematic.
Now, this is still quite experimental and conflicts might exist between dragging the window and some other mouse actions for some applications. I hope we can identify, and fix, most of these conflicts in KDE SC 4.5 beta cycle, so that a well crafter and fully functional feature can be delivered to users with the first release candidate.