I’m not sure if Mage or Rogue are harder or not. What I know from my own experience is that Rogue and Mage were always more unique due to their playstyle difference. Positioning as mage is way different than on any other caster and playing rogue is nothing like playing any other meele. These classes are very unique in playstyle and they need more effort. Switching from Shaman to Warlock or any other caster than Mage is not a big deal. Same goes in case swapping from Enh to DK or Ret.
Warrior was 3rd unique class back then but it changed since they removed a lot of Warrior mechanics and skill from spellbook and since they gave ragelike resources to most of specs.