It seems even the slightest graze about the torso is enough to kill you off when someone strikes you with a lightsaber. Anyone besides me find this strange ?
My guess is that is reality, a lightsaber stroke would usually cause pretty spectacular dismemberments and disembowelments, but that would give a pretty hard-R rating. So Imma say it's just cinematic shorthand for more spectacular deaths.
I've always viewed lightsabers as knifes, only hotter. If you get stabbed it will cause a lot of damage from blood lose and damage to the internal organs. Same with a lightsaber along with burning from the heat, you'd probably get a lot of bleeding and internal damage. The picture above shows where he has been hit, but doesn't show how deep the wound is, for all we know that strike could have been a glancing blow, or it could also have gone half way in slicing him open and slicing his internal organs as well. Also as Strongbow says the rating limits what we see in the film. Kit Fisto actually gets his head chopped off by Palpatine, the reason it was changed is because GL thought it was too violent.
It's like people in movies and gun shot wounds. In movies, a gun shot anywhere on your body kills you instantly. In real life, that may happen sometimes, but a gun shot wound does nothing in a lot of cases but cause you an agonizingly slow death. It's also like in action films where the hero delivers a single punch on any part of the body of the henchman he is fighting and they are instantly knocked unconscious. It's just a thing that movies do.
With guns, and probably the same with sword strikes, how long you take to die is dependent on where you get shot, or in this case, sliced. You're gonna go down a lot quicker if the bullet, or blade, his a vital organ, than you would if you get shot in the stomach, or if you just cut into the fat. Frankly, only one Jedi we see in this scene is human, we shouldn't assume all aliens have the same internal anatomy as us. Kit Fisto could easily have been struck in a really vital internal organ there.
It is a concentrated laser beam that will slice through you with no effort. But the real issue is that on ROTJ, to maintain a PG rating since PG-13 didn't exist yet, most of the blows were pretty tame and didn't look that gory. With ROTS, due to the PG-13 rating, Lucas could do more but opted to not go over the top. He only did so on TPM, but most of Maul's demise was off camera and pretty bloodless after the initial spray.