@X-Ray
In another, much older thread, you said that the bullet through the head thing was often fatal due to the loss of blood pressure in the brain rather than the immediate damage to the brain tissue itself. I would assume that decapitation would cause this effect every time, since the carotid arteries would be totally severed. The worst thing about decapitation by sword (other than the grim spectacle), I imagine, is when it takes more than one stroke :-/
As for the metal plates, think of the cleaning problems...
To be fair, *if* you must execute people I guess that the most humane way would be to gas them with an anesthetic until overdose - or administering a heart stopping drug once fully under.
There'd be no risk of misinjection into body tissues, no frying, no slow strangulation at the end of a rope, no struggling against the first wisps of bitter almond, no repeated blows with a blunted sword, no waiting for the firing squad. Just sleepiness then oblivion...