A little something I posted over at GitP:
Levels are, in fact, tangible entities in the World of Prime. When you defeat an enemy, you consume his soul. Every discrete number of souls grants a distinct suite of powers.
Thus, the leveling up process is a perfectly concrete experience. It takes a day to manifest, and at the end of it you have the added vitality of another life (hit points), plus supernatural bonuses to combat reflexes (BAB) and possibly spells.
Some classes, like Ranger, require you to actually study skills or stuff, but only to become 1st level. After that you get supernatural bonuses to your skills.
What we role-play is the increasing of attributes. My players started out with all 10's, as they were originally peasants. Over time they escaped the dulling oppression and bad diet, and their stats began to rise. When they got their third level or so, they rolled (using the 4d6 method) their attributes, indicating that they had enough experience to have fully developed as Heroes. (I wanted them to keep role-playing stat increases, but they preferred to gamble on good rolls).
The difference between a professional warrior and an amateur in D&D is the difference between a STR of 14 and a STR of 10. The difference between the toughest guy you know (or know of) and a wimpy geek is the difference between a CON of 18 and a CON of 8. Training and experience can make you stronger, tougher, faster - and even increase your willpower, observation and thinking skills, or force of personality.
But casting a 1st level spell - that's not something you can learn by doing. That's a supernatural ability, full stop. Jumping out of an airplane and walking away from it? Supernatural. Raising the dead? A supernatural ability gained only by enslaving the souls of thousands of the dead.
It has to be this way. Otherwise everybody worth a peanut would be a 9th level cleric. It's all fine and good to say, "some people are doctors, and some people are ditch-diggers," but the only difference between those two classes are how much money you make. There are plenty of very intelligent, capable people who settle for ditch-digging because they like it.
But no one would settle for not raising the dead, if it were possible. Plenty of people would do whatever work it took to gain such a powerful, desired ability. So the existence of levels has to be governed by something other than personal desire - otherwise you're saying that your world is comprised mostly of losers who deserve to get eaten by gnolls because they didn't try hard enough.
My world is full of realistic people who do everything they can to survive and thrive. Sadly for them, the rules of the world mean that many must die for one to be promoted. But the existence of monsters means that heroes must be promoted, or all of humanity will become gnoll-food. The World of Prime revolves between the twin horns of this terrible dilemma.