Conaill said:
That argument is rather irrelevent, because this is exactly what sneak attack is trying to model, i.e. an attack on an unspecified but more vulnerable location. So why can't a Rogue do a sneak attack on, say, a construct with a regonizeable anatomy, but he *can* sneak attack some crazy aberration from the 7th layer of hell with a discernable but unrecognizeable anatomy?
The theory is that a construct has no "anatomy" in the sense it has no parts; it's generally a heterogeneous mass maneuvered by magic. Undead have "parts" but they are held together and operated by magic forces, hence the parts are irrelevant as long as they are present.
Here's a comparison: a rogue cuts a human's achille's tendon. He's just lost most of the mobility in one leg from a rather shallow cut, less than 1/2" deep and 2" wide.
The rogue makes the same attack on a zombie and a skeleton. The skeleton's tendons are just hanging there, if even present, so he really just nicks the bone. The zombie's tendon snaps but the muscles aren't driving the body's motion so it's loss is irrelevant.
The rogue then cuts a stone golem's leg. Since the leg isn't severed, it continues to function with no loss in mobility under the magical forces that animate it.
Suddenly, a demon pops in. The rogue, apparently a ground-hugging sneak, slashes at the flexible spot on the back of a joint on the leg. While not necessarily the achille's tendon equivalent, it still seriously inhibits the demon's mobility as the leg's function is degraded far in excess of the raw damage dealt.
A nominal rule is that all creatures with functioning internal organs and musculature have some general weak points: the inner side of joints (i.e. elbows, knees, armpit, groin, wrist, ankle, neck), and sensory organs. This holds for vertebrates and invertabrates in most cases.
Ooozes and the like have self-sealing membranes and are pretty darned hard to sever as the material will reabsorb if it comes in contact with itself. Puddings are nasty since all you do is create a smaller blob to help flank you.