Well if you wanna add on and ask that, then let me add on to it as well. Why didnt it kill Cloud in the Nibelheim reactor? Cloud gets it driven through him and all that happens is he throws Sephiroth off the side. Clouds was pretty much the same as Aerith when she got stabbed. Granted Sephiroth had some velocity from coming down from nowhere, but still the same wound area.

Some of it could just be plot inconsistancies. I'm pretty sure that the Nieblhiem incident was added later during the production of the game.

I once read a theory that Sephiroth's masamune was a holy sword and it would kill everything it touches. Well ok, so it didn't kill Cloud, Zack, Tifa, or Tseng (according to AC). But perhaps it instantly kills anyone it completely pierces?

This theory also explained why there was no blood during Aerith's death.

Also, if we disregard that theory, it did completely pierce her and most likely tore some major organs. But with everyone else it only seemed to swipe them.

I admit, that theory would probably work under different circumstances. I mean, there is an actual legend that states that the Masamune is a holy sword and that it doesn't draw blood from someone who is righteous/pure of heart. However, the fact remains that, in the Nibelheim Reactor, the sword completely pierces Cloud, yet he survives the incident. Within the FFVII Universe, the Masamune just seems to be a normal sword. It probably killed Aerith because he pierced one of her vital organs, whereas, when Sephiroth stabbed Cloud, he just happened to miss his. As for the blood...You have to remember that this was a PSX RPG. First off, they probably wanted to keep the game's rating down to T. Graphical limitations probably had a lot to do with it, as well.

It's pretty much an established rule when it comes to JRPGs that Gameplay=/=Storyline. You can't honestly believe that Cloud and co. can survive multiple Super Novas when, in Advent Children, Cloud was put in a state of near-death for over a day when Loz and Yazoo hit him with that explosion. There's really no other explanation. Think about it. That means that even Cait Sith (a doll) and Barret (a normal human) are capable of withstanding several direct hits from a Super Nova. The same can be said for pretty much any RPG character. In Gameplay, it's true that the party appears to survive being stabbed, burned, zapped, frozen, and crushed by giant monsters, but as far as the storyline is concerned, they just dodged or blocked most of those attacks. However, Sephiroth's Masamune really did pierce Aerith in the storyline, so she took actual damage from it.

Actually, Sayuri, when Cloud was stabbed at the Nibelheim Reactor, he didn't have any Mako in his system. Hojo didn't test out his Reunion Theory on Cloud and Zack until after the Nibelheim incident, so I don't think the Mako would've had any substantial effect on Cloud's recovery from the stab wound. However, the fact remains, Cloud was still a well-built young man, whereas Aerith was a petite flower girl. Even if Cloud hadn't been enhanced by Mako yet, he did say that, physically speaking, he was qualified to enter SOLDIER, so he was obviously in very good shape for a normal human being. Meanwhile, Aerith never displayed any physical abilities above those of the average girl her age, so it's pretty reasonable to assume that she's not quite as durable as Cloud. As for Tifa...yeah, she wasn't stabbed, she was just slashed across the chest. For all we know, the wound really wasn't that deep.

On the subject of the blood...I don't really know why there wasn't any blood. It would've been a bit more graphic to show Aerith getting impaled than it was to show Sephiroth with a bit of blood on his forehead, though, so I think they just intended for us to suspend our disbelief. Besides, I somehow doubt that Sephiroth is wielding a sword that's incapable of drawing blood, especially considering it can be seen with blood on it in some of his concept art.