I sometimes wonder what RPGs would be like today if OD&D hadn't included the concept of levels or indeed any kind of experienced-based advancement system. With the exception of Traveller in its original form, I'm hard pressed to think of any RPG that doesn't include some way for characters to improve over time. I don't necessarily think any kind of improvement is an inherently bad idea -- obviously not, given my love for D&D -- but I do think that the idea of gaining "experience" through adventuring is one that's sometimes had a negative effect upon the development of roleplaying games, making them all, ultimately, about becoming more powerful over time, with "power" being defined largely in game mechanical terms.
In the aforementioned Traveller, your PC begins the game as skilled as he's likely ever to be. Acquiring new skills or improving existing ones is remotely possible by going to school, but it's a slow process, as in the real world, and it effectively takes the character out of play for long enough that very few people ever bothered with it. Consequently, "advancement" in Traveller was measured by the goals a character achieved, whether they be becoming a rich merchant prince, exploring a new world, or establishing a potent mercenary company. Players became better at playing their characters over time, but their characters didn't become mechanically more effective as a result of having played dozens of adventures. This made it a lot easier to create adventures, as there was no mechanical necessity to "ramp things up" when dealing with characters who'd been around a while.
D&D just wouldn't be D&D without levels and the concomitant increase in mechanical potency. I have no particular interest in changing it, but I do think many of the frustrations I have with, say, adventure paths, are a direct result of the rather stark differences between low and high-level characters in latter day versions of D&D. That's why I prefer versions of D&D where the rate of advancement is both slow and less steep; it keeps things closer to a "human" level and makes it easier to avoid the temptation to turn everything into an epic struggle. Still, I do wonder whether a RPG without "leveling up" in some form would even be able to capture an audience nowadays. Is the concept so integral to what people think of when they think of roleplaying games that they couldn't imagine a game without it?