A DH that can use retreat, use it again mid-air, then turn around and leap twice; while a hunter can disengage once, is bad enough. A DK is pitiful in comparison to the hunter’s movement, to the point where Gally pugs I was in were turning down DK tanks because they just knew there was no way this guy was running out with his turn as the bomb.
All classes should have their strengths and weaknesses, but movement should not have such a disparity that one class can run from one end of a dungeon or BG to the other 5-10x faster than the slowest.
The fix, at least for hunters, is to give them the same ability to double up on disengaging the same way a DH can, if used within a few seconds. Maybe a DK could use one CD for the speed increase and if they use it again with the second charge, it stacks the increased speed the way a Monk can?