I think they should go with the idea of a canine infected who clamps on to a survivors leg and slows them down. Not dragging them, but greatly reducing their movement. And it would be nice if it could drag a incapped survivor because no other infected has that ability yet, which is a little dissapointing. You can be carried away while your able to run, but not dragged away when you don't even have the strength to stand back up, let alone defend yourself from an infected attempting to smash you into the ground? That be messed up.