Let's start with a screenshot:
In this game, you can choose whatever class you are, just by clicking a button. Think of it like dual specs, but then for all classes.
You have two XP bars:
Left (purple): your character level. Increase to get better weapons, weapon requirements, etc...
Right (green): your class level. Increase to get more class skills, etc...
I don't post this as a suggestion to WoW, if not only because I'm not interested in bringing that to the game. But I do want to share the idea, and look what you guys think of it.
*BTW: the game is AQWorlds, just if you were wondering...
** How do I make that screenshot smaller so that it fits into the screen on smaller screens without the need of horizontal scrolling?