My last wizard character was a gangly kid who kept trying to pull off wizardly hauteur without possessing the age or wisdom normally required. Since wizards must have beards, he had coaxed a few pale scraggles onto his chin. No one would have called it a beard. Since wizards must have pointy hats, he'd bought one. Being brand new, it completely lacked the moth-eaten look, and did not flop properly. He kept tripping over his robes. And it's awful hard to make words of power sound earth-shatteringly impressive when your voice owes less to Christopher Lee than it does to Tobey Maguire - and still cracks once in a while. The harder he tried to be taken seriously, the more laughter he drew. (Yeah, I know, sounds like Rincewind. In my defense, I drew up the character before reading Discworld.)
My last paladin had been exiled from his order over a doctrinal disagreement before the game began. Rather than a shining example of potential, the (corrupt) head of his order used him as a cautionary tale, a warning to those who would question canon. My guy's struggle was not to bring light into dark places, but to figure out a moral course of action that would allow him to remove the corrupt leader without destroying the order.
My last rogue was lawful good. He worked for the crown as a counterintelligence agent. Rather than getting in trouble due to a complete lack of scruples, as your average chaotic-neutral rogue would, he had too strict an ethical code to perform his job in the most expedient manner.
I wish I'd understood when I was younger how much fun it is to pervert or ignore stereotypes. I used to play dumb door-kickers every time.