There's currently the option to move windows between displays with the Shift+Meta+Left/Right arrow key shortcut. 

In the context menu of the taskbar there's an option to move an application (or its window) to a different virtual desktop. It would be convenient to have such an option for moving windows between screens.

Reasoning:
1) Grapping a window with the mouse and dragging it all the way long can be cumbersome.
2) If you cannot see the secondary monitor/beamer, because -- it's placed at a different location than the main monitor your are working with or it's turned off or KWin did not notice that the secondary monitor was detached (happened on X11 last week to me!), stopped working -- or whatever, it's hard to grap a window over there.

In this case a context menu entry "Display application on screen [number; display name]" would be very helpful. :)