I'll try my best to cover how it worked in in Zeal, whereas the Heroes plug-in first started officially.
There were 5 classes: Warrior, Healer, Rogue, Crafter and Caster.
Like now, each class had 4 different specifications. Most of them are the same now, but with crafter you only had Smith, Mason, Alchemist and Engineer. Some skills we have now are ones which we used to have, but overall many new ones have been added.
Leveling your main class took you to level 50, not 30 (level 50 was the max level in Zeal). When you mastered the class, and hopped onto a specification, you had to reach level 50 again to master. This took longer because all exp was halfed.
To gain experience, anyone could kill MOBs, mine, ect, unlike now whereas only professions get experience from mining.
When the 1.8 update came out, with many new features to the game, I guess it was time to allow more specifications in the Crafter class, which eventually turned into the professions we had today. This let normal PvPers have the abilities of builders, and vice-versa.