1. Forces you to full screen on the main monitor regardless of which monitor the window is currently in. So if you click it on your left hand monitor it jumps to your right hand monitor.
2. Forces you to look at linen (!!!) in the other monitor. It doesn't even do the slightly less broken behavior of giving you black on the other monitor, it literally fills it up with that ridiculous linen repeated pattern so that it can optimally annoy you as you are watching the movie. Not to mention that you should have the option of actually using your other monitor for actual work.
This is why I just can't use iTunes or Quicktime for watching video content anymore. Because someone literally went in and deleted working code. Quicktime used to have nice options for blacking the other screens or not, etc. and they were taken out. Were people demanding this feature in VLC or something?