@Martin Peres, I've done a little bit of investigation. 
 So if I don't use any desktop effects in kwin, the video on mplayer+VDPAU is
choppy and not visible sometimes (except fullscreen). Also in this case I
cannot exit fullscreen. Basically double click doesn't have any effect as well
as alt+tab. Or to be more precise it seems like a window is minimized but the
video is on top of desktop. 
 If I use desktop effects+Xrender everything works like a charm. 
 The desktop effects+OpenGL render behave exactly like you described in your
message. 
 By the way I use KDE 4.10.5 (OpenSUSE 12.3). Is it possible that this issue a
bug in kwin? Which options do you use in KDE?