The Mage is the comfort class. You conjure your own food and water so you never spend a copper on consumables, Blink and Frost Nova get you out of anything, and Polymorph turns a deadly add into a sheep for thirty seconds. New players gravitate to Mage and they are right to. It is forgiving where it counts and powerful where it matters.
Where it gets interesting is the gold. A Mage with the right AoE build can pull half a dungeon, freeze it in place, and Blizzard or Arcane Explosion the whole pack down at once. AoE farming spots like the Maraudon princess run, Zul'Farrak, and Stratholme are how a lot of people fund their whole server. No other class farms gold the way a Mage does.
And then you raid, and you are at the top of the damage meters anyway. Frost for the survivability and the early Winter's Chill debuff, or Fire for raw ceiling once you have the gear and the hit. Either way, you press buttons from thirty yards away while the Warriors bleed.