Elana has it correct. A couple of thoughts. First, I think when you see a still shot form behind you will find the horse is either very cowhocked or base narrow. If you watch the vid, especially as te horse moves away, you can see where his hind feet track nearly in directly in front of each other. The rider also has the horse in what we refer to as a false frame. Sure, he LOOKS like he is on the bit when in actuality his head is in position but all the effort is on the forehand...she is not letting him stride out and he has no impulsion from behind. What this makes me wonder is just how much energy he may have that she is containing him.
I'd also like to see a more back to front riding. Even on the forehand he looks like he has decent movement but she is riding him with her hands so low to make it look like he is all calm and cool that she is sabotaging his movement.
Maybe terminally paranoid after seeing so many but I also think I see a little head bobbing going while he is tracking left..could be just the rider's hands and her constant asking for him to be on the bit but he doesn't look 100% "sound" even.