[SixOfSpades]

Of course, the most realistic system is to give EVERYBODY basically the exact same abilities, and then, the only way to become more skilled at those abilities is to practice them. So, for instance, you're a Fighter/Cleric/Thief/Mage, and you spend your day whackin' bad guys with your axe. Bang, you've gained a level as a Fighter and your skill with Axes has increased. But if you had spent that same day sneaking around, breaking into buildings and stealing things, your Thief level would increase and your Lock Picking and Stealth scores would go up.