Amy and I started watching Hellboy (the director's cut) today at lunch, since she's never seen it before and we are planning on seeing Hellboy 2 soon. As I watched Hellboy get knocked and go flying through a series of museum display cases, I was reminded of a pet-peeve of mine: the anti-gravity fall. It occurs when a character is punched, kicked, or blasted enough force to send them flying 20 feet or more, and that flight-path is an unnaturally slow and straight line.
The flight and fall look horribly fake because the character travels in a straight line for those 20 feet before suddenly dropping to the floor and sliding. Everyone knows how objects look when they are thrown: there is an arc as gravity overcomes the force of the push. If the director's intention is to communicate that the force was so great that the character actually flew 20 straight feet, than that force needs to be shown. The characters should fly with much greater speed and hit a wall really hard. If the intention is just "a pretty good kick," then show an arc to the landing.
From memory, I know I've seen it in Hellboy, X-Men, and The Matrix. The specific scene I'm thinking of in The Matrix is when Neo kicks Agent Smith down the hallway. He travels straight back at a constant height above the floor before finally dropping at the end. He waves his arms as he flies, and the camera follows him on his straight path. Rather than selling a supreme and forceful kick, it reminds you that a stunt team was dragging him in a harness and an effects team erased the wires. Why otherwise competent action directors keep making this mistake is beyond me.