You should always be able to shoot someone in the head and fold them. But IMO it happens too easily and it is too often that you just hear *crack* and keel over. And when that happens, the fight is over and the fun is over. The possibility of this happening keeps the game exciting, but it doesn't need to happen as often as it does.
Traditionally whenever I mention this, I get downvoted to shit, and honestly, I don 't really get it. I know everyone likes the feeling of dropping someone with a one-tap, but does everyone really think that they should happen as often as they do? I think over 50% of my deaths are from headshots, and at least 50% of those (probably more like 80%) are the first bullet fired in an encounter.
I bet if you start writing down how you die after each raid, you'll find that it is most often to headshots.