The reason for the casting time is to make players sure that they don't have aggro when they call the mount. If you do have aggro, you can't call the mount and if you're attacked while calling a mount the casting time is interrupted. Not sure why they carried it over, but the notion here is that mounts aren't 'safe mode' and Heavy is meant to instill a sense of 'dread'. While this was true in 1.0, that just isn't the case in 2.0. I'm more inconvenienced than scared and if I aggro while on a mount I just keep going. While insta-cast isn't something I can get behind, Heavy needs to die.