Well, if you haven't been on since Wrath of the Lich King, you have missed a lot! First of all, there is a new continent, Northrend. There is also a lot of new dungeons and a new class, the Death Knight (fun class in my opinion, have to have a level 55 character to unlock though). There is going to be cross-realm dungeon grouping in a bit, and there is also the new Dual-Spec ability (allows one to have a second set of talents, costs 1000g though...)
Mount status is also different, one can get a mount at level 20, then journeyman riding at 40, and a flying mount at 60.
As for a mage class, I got mine to level 36, and in my personal opinion... they take up WAY too much mana WAY too fast. That should be fixed soon though, since they are lowering mana costs for spells soon, I think...
Also, they are lowering mana cost and raising health regeneration for lvl 1-10, and giving rogues dual wield immediately.
I am not even going to start talk about cataclysm, that info is everywhere!
So, that's about it... sorry if I missed anything.