Its alot easier to make a soft rock-paper-scissors balance system in RTS games though. Applying SC2 balancing to the MMO sphere gives you auto-wins and auto-loss. Regardless, he didnt say he wants auto-wins or to be the OP class, he said the game is less fun if everything is watered down into a perfectly balanced, bland soup that tastes like nothing. And that's what most MMO PvP that was "balanced" would turn. I say would because they are never was balanced proper to begin with. Now maybe you can pull examples out of the air from the more recent years when every class in WoW or any other MMO since was balanced in several PvP scenarios at the same time (arena, small bg, big bg, not OP while holding a point, unstoppable runner etc). I just know when I played there was always a class or two that stood above the rest in too many of those scenarios and and one or two that were the laughing stock of the other classes. And these balance attempts that didnt actually work out came at the price of interesting pve abilities.
World pvp might be a problem with this suggestion, but for all the restricted to pure pvp areas like arenas I would prefer classes to have a different skill pool. You can have overlap for skills that actually work the same, but if in doubt split them in two. Different names and all so the whole "works different in pvp" issue doesnt come up.
Limited skill bar is great for PvP (well for PvE too). Remains to be seen how often you can switch out but I hope it requires being out of combat for a long enough time that switching is a preparation decision and not one made during a fight.