the lightning damages the body by superheating the bones, which is why vader's bones glow. It melted the prosthetic he has at C3 (3rd vertebrae on his spine, where his neck is broken, again this info is from the enhanced shot from TFN) so he had difficulty breathing and could not move without Luke's aid. He needed a high pressure, medicated atmosphere to survive after inhaling volcanic ash. By removing his mask, luke euthanised him instead of having him go through the slow and painful ordeal of superheated bones and a broken neck.
again Darth Jello, info from that other site is not info from Lucas himself. and that whole pressure thing is what "his life support failed" meant in my previous post. the lighting killed the suit, the suit kept Anakin alive, no working suit, no working anakin. It wasn't the lightening alone because Luke took just as much and was okay.
ok look. i don't know how many times I'm gonna need to repeat myself here, so I'll just be blunt. I apologize in advance if this is offensive. That is THE MOST inconclusive CRAP evidence to support ANY indication to what Anakin's injuries will be in Episode 3. There are too many filmmaking factors (screw ups, guess work, flaws in the final product as opposed to storyboard) to make ANY judgement based on that. All that scene means is he is getting ELECTROCUTED TO DEATH. It wasn't meant to give you a glimpse of injuries and as such can not be treated as evidence for his injuries. Until III opens, or the really good spoilers hit, we will have no good evidence to what it is that happens to Anakin, too many major scenes have been given new context because of the prequels to take that little glimpse of his skeleton any creedence. BAM!