Considering the amount of physical damage a mid- to high-level character would require to be taken down, it makes no sense that it would all be physical damage. Gygax explained this all decades ago, and it will apply so long as a massively abstract system like hit points is used.
I'm not talking about what Gygax thought; I'm talking about what happens in YOUR mind's eye. Often times what we intend to say is different than how others perceive it; game rules vs. fluff can be seen in the same manner. How do you honestly perceive hit points?
Depends entirely on the creature hit and what attack it was that hit. On a bulette every hit is a proper wound, on a pixy, only the lone that takes it down. PC's are in the middle, but with large variance.
The halfling rogue mostly aquires fatigue, minor bruises and luck running out, the large in-your-face barbarian starts of with aquiring cuts and goes all the way to arrows and large gashes all over his body.
An attack that goes under 0 is a potentially mortal wound by default, unless other options (and optional rules for going to 0) are explicitly agreed by the group.