They had a unique skill/mechanic each. Though I'll agree the "class" system in FF6 wasn't the strong point, it could have done with a more traditional job system, or more depth to their unique traits. Still for most of the game you had unique characters, the only time you reached "everyone is the same" is when you started grinding way past what you needed for the story. Which I guess could be pretty early if you were in that mindset.Make sure that every character can learn every spell so that all of the characters are the same at the end of the game.