What is he??

Well one thing that doesnt add up to me at least.. what part that surgeon played.. is David even the guy in the photo or some other guy... maybe he just had some plastic surgery.. Maybe it was for something he had, like burns ect.

The increase in human knowledge is the cause of the decline of religions.

The perfect killing machine. Barely human. Obvious regenerative abilities, even after death. Programmed and coerced mind. I have no reason to believe his power was anything close to a terminator, though his ability to regenerate tissue was better.

No, I'm in Touch with humanity!
- Patrick Bateman, American Psycho

LMAO @ the replies.

Well, Luke stabs him directly in where a normal person's heart would be... and he walks away from it.

So something, whether it be some Wolverine-like regeneration or the guy's a cyborg would certainly be appropriate.

However, the script calls for ambiguity, so ambiguity is what we have to work with.

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He was genetically engineered but could have had some parts cyborg to him and in fact I bet he does. He could still be killed tho he just probably heals real fast so you'd have to shoot him in the head or blow him up. Just my opinion, id have to ask the director and he probably wouldn't even tell us definitively.

A really really bad actor. He could play Santa and look dead inside like a robot, really.

For every lie I unlearn I learn something new - Ani Difranco

I think the vagueness of this worked in the film's favor because it's otherwise pretty standard fare for the subject matter. In other words, I figured out the basic trajectory (and some of the details) in the first 10-15 minutes.

The theory that he is actually Caleb is an interesting one, but I don't wholly believe it because of a couple of little details in the film (mainly the military head stating that David faked his own death, went awol, and was programmed to kill anyone who might compromise his identity).
I suppose those things could have been said to throw the family (Anna) off-base, but there was nothing at all to support this stance and, by the time this information was revealed, it would have been pointless to cover it up anyway because the damage (so to speak) was already done. There was also absolutely no indication from any family member that they even remotely suspected that it could be their son and you would figure they would be the ones to pick up on that stuff... unless they were all a bunch of total idiots (which also had little to support it, and creating an idiotic character onscreen is a pretty easy thing to do).