Hmm I would have to disagree a bit here. I also like the idea of classes and the idea of set roles (seeing as I am adamant about playing support roles exclusively in MMOs), but I think the prospect of having new, more dynamic ways of working together is just as exciting.
I liked the example you brought up of FF4 vs. FF6 because these are also two games I love dearly, but the reason FF6 wins out for me (besides the story and the characters) is because if this freedom to choose what you would like to compose your group of, and have the ability to always change your configuration when you see fit. In FF6 certain characters still have unique abilities, e.g: Celes with her Runic, Edgar with his Tools skills and Terra with her Morph/learning spells by leveling, so each character does have something unique to contribute. In addition, you have the choice of selecting which espers to attach to each character to determine what spells they will learn and what stats will improve when they level up, essentially giving you the choice as to who will be what class without affecting their unique skills.
Such is the case in Guild Wars 2. You have a class with a unique set of abilities that can allow them to be more versatile according to the situation. For example, if I have my elementalist, I can do AoE damage with a certain weapon set and attunement, and if I see enemies coming to close, I can use my close range skills to defend myself without having to depend on someone having to take the monsters off me. Also, if need be, I can switch to healing to help out my allies if they are in need of recovery skills.
Guild Wars also adds the cool idea of combining skills to help each other out, for instance if I cast a fire wall it will keep enemies away, and allow rangers to shoot arrows through it to become fire arrows. Or, I can freeze the floor and a warrior can break it to help us all gain ice armor as a form of defense.
In this model, we are more able to choose a class we like, and play it many different ways, still have the option being more defensive or offensive, and ultimately have unique contributions to help in ever changing situations. Finally, I feel like this system also makes it so that if you lose one character, the whole party/battle does not fall apart because someone can temporarily take up that role while the fallen ally is being ressed. The incentive to revive that person is welcome them back for their unique role, or in the elementalists case, the AoE damage, defensive skills and the ability to do damage while continuously healing.
Wow that was long
But I wanted to express what I liked about the game from the perspective of someone who was just as skeptic as you Roll. Those are my two cents
*edit* Added paragraph breaks for you because @_@ -Bear