Do what one of my mates did and sign up for multiple accounts, play both sides, play all classes and then roll a...
Due to the single job aspect of their roles a Mage is probably good to play first up to about 10 or 15, then re-roll a more varied class.
If I was to start all over again, I'd probably go for a druid (to play all 4 roles) or a Paladin (can fill 3 roles). When Cataclysm comes, I'll roll myself a Worgen druid to explore the new Old World and a Goblin too (though probably on a different server). Gobbos look like they have an intriguing opening few levels like the DKs did. Overthrowing the mine, fleeing the eruption, fleeing the flooding, throwing their lot in with the Horde.