There will never be "boom head shot" in HD for, without limitation, the following reasons:
1. the way things are set up now, a head hitbox will require dozens and dozens of lines of ad hoc spaghetti just to make sure there's never any chance a monster (or worse, a player) can shoot themselves in the head trying to aim at an unusually steep angle. Without this separate hitbox, there's no way to make the head a smaller target and the "all or nothing" gamble one takes when aiming for the head no longer applies.
2. If the head, then what about major arteries in the limbs? The hepatic artery? Gut shots, knees? The spine? To carve out this special treatment for the head while treating the entire rest of the body as homogenous would create a pointlessly lopsided target.
3. Related to that lopsidedness, we live in a culture that far too greatly values the chattering noise in the cerebrum above other things that matter at least as much in our bodies and environment. Nevermind realism, mythically it's not something I particularly feel like supporting at this stage of my life.
4. Inasmuch as HD is a simulator for best practices, a bonus target for center of body mass is preferable as Somagu has mentioned.
5. Inasmuch as HD is a game with aesthetic considerations, a centre target that roughly conforms to the golden ratio on the vertical axis is preferable to one on the top 12%. Also, rewarding players for winging their targets and penalizing them for hitting dead-on seems a bit too weird.
6. I am no longer able to associate headshots in games with anything other than prurient gorefests. While fun, it's not something I'm at all interested in replicating for HD.
The only One exception to all this is the fist. Not entirely sure why I don't feel similarly about melee weapons, really, though punching someone in the face is always more personal than blowing their brains out from afar, and the accuracy issue is very different - you can miss the head going forwards and hit them on your way back, which obviously can't happen with a bullet.
As for point 6, the ridiculousness of gibbing someone by punching them is one of my major formative memories of first playing Doom, so it needs to stay.
EDIT: The other exceptions of course being the pinky, lost soul and caco...
