You're sitting at your character creation screen, starting a brand new game, maybe with no previous gaming experience at all. Or perhaps you're well-versed in the realms of MMOs, and you have a favorite character archetype already. No matter which applies, we all started at that screen, wondering: What class should I pick? What does class choice even mean? What are tanks? How come this class's armor is so hideous? What on earth is a paladin?
For me, I was devouring the manual that came with vanilla WoW as it installed, trying to find a class I'd like. When I stumbled upon druids and the fact that they could turn into bears and kitties (and moonkin, although I didn't know that yet), I was instantly hooked. Later, in the 40-49 bracket of Warsong Gulch, after getting wrecked over and over by warlock DoTs, I thought, "Hey! Warlocks seem OP! I should make one!" And thus, the warlock was born.
A guildmate of mine chose a rogue because she'd watch her husband play and he would always exclaim how annoying they were and how much he hated rogues. His anger amused her, so rogue she went!
What influenced you? Death Grip looked awesome? You wanted to dispel Hunter's Mark because you hated that graphic (my priest!)? You wanted to shoot huge balls of fire at people? Tell us about it!