(Oh, If I am wrong about anything that I said, let me know. I have never played VI you see....)
Well, what you said about kefka and geshtahl being evil without any background as too why is correct. though, sephiroth is somewhat overrated. Terra from FFVI has a very similar history to Sephiroth, she is half human, and half esper. Human's and Espers never got along, there was a war that enarly destroyed the planet, and the empire, the last time they crossed over to the esper dimension killed and took many esper corpses. One of the casualties was Terra's human mother, who was killed by geshtahl so that he could raise and turn terra into a living weapon with no emotional ties or understanding of what it was to be human. See the similarity, the difference is that sephiroth knew kindness, respect, admiration and friendship. Terra knew none of these things, she was raised since childhood to kill. When she began to question why, she was fitted with a mind control device that subdued any free will she had.

I guess this is why I consider Seph so shallow and overrated. Terra should have been just as angry as he was, even more so when she saw the remains of her father, and was told what happened and how she came to be in her situation. She held onto her humanity in situations that were even more dire then sephs lifestory.

Cain from FFIV is also a fairly deep villain, he switches from hero to villain at certain points in the story. Initially he is the main characters closest friend. In fact, there are allusions that he helped to train cecil. Yet at one point Cain and Cecil are seperated, and when they reunite cain is now Cecil's enemy. The main villain didnt so much brainwash, cain, as brought out his darker side. Cain always loved Rosa, yet She loved cecil. A person Cain saw as weaker then himself. The jealousy and pride was Cain's downfall and almost killed him.

Also one must wonder if you played FFX. If you want background story on a villain, it doesnt get any deeper and more tragic then the final battle in that game.

I would also reccommend you hunt down Xenogears, One of my personnal favorite RPG's ever with a story that is by far deeper then VII's, deeper then even FFVI, and with a villain who is at the same time more dispicable then kefka, and more tragic then seph.

And as for recapping the story of FFVII in the first ten minutes, AC HAD a story, then yes it would have been neccessary. There are more then Seven major fight scenes in Advent Children. Thats less then your average Jet Li/Jackie chan kung fu flick. In fact, I cant think of any martial arts film that has that number of major fight scenes. Then there's major story bits... I swear I cant figure out what tifa see's in cloud. He's moody, antisocial, selfish, stupid, shallow, bi-polar, ect. I think we all know the two biggest qualities cloud see's in (or would that be "on") tifa, even more so considering they are bigger then ever in Advent Children :rolleyes2. Then there was clouds relapse into depression, after I paid good money to watch the mook work through it all in the first game. and the villains themselves were utterly shallow with no backstory whatsoever. And no the fact that their backstory was included in some novella doesnt count. If the film makers cant be bothered to put some story elements into a movie, then the story deserves to be bashed. Basically AC was one big sugar rush punctuated by a nonsensical, hole ridden plot and some good bits with rufus and the turks.

By the way, speaking of masamune1600, Im still waiting for your reply