@Ben F

My dear friend(fail opening)
The problem is that you have got the whole thing of continuinity wrong. When you make a new character you don't in fact where was the story and the storyline when the game had first begun but the storyline of your new character starts where the storyline is now. Whichs means: When you make now an alt the dark portal is open already and people have alreeady taken the fight to Lich King. When the cataclysm is released the storyline reason why you first decide to outland then northred then deal with deathwing is a matter of personal choic of your character. You first decided to deal with the smaller problems first then th bigger ones. And in fact cataclysm isn't what happens after artha's defeat. It's what happens a couple of years after people decide to seriously take the fight to lich king but for you they have all failed to kill him. So when cataclysm is released arthas will be still alive(it isn't mentioned otherwise anyway) and you will have to kill him. Don't forget that all the victories in all instances and raids have not happened for you before you clear them, the only things that have happened are the opening of the dark portal, taking the fight to arthas and the world suffering deathwing's devastation. But before you complete any instances or raids yourself no victory has happened for you. Arthas, Kil' Jaeden, Illidan and Deathwing are still there.

I hope this answers your question.