1. The title bar of a QT X window seems to obscure part of the image. It will fade away revealing the portion it covered when you mouse away from the window. Fading away is a large problem. When sitting with clients looking over versions of an edit, I can already hear their confusion as they can no longer see the Title plus Version Number plus Date of the two side-by-side clips we're watching. Editors and clients need to see which playback window is what version at a glance. Clients won't like this.
2. The floating controller window does not resize when you resize the video's window. This can lead the floating controls covering a large portion of the video.
3. Just the fact that the controls are over the video is an issue. Any time you want to adjust the video in some way, some portion of it will be obscured. I'm sure this will reap havoc with subtitles.
4. When you begin playing a video the controller is there for a few seconds no matter what, again, blocking part of the image.
5. QT X cannot play all open videos simultaneously apparently (I haven't tried it yet, this is from an email), which makes playing multiple versions of a video simultaneously to compare them impossible.