First, for those of you not familiar with the Buffy series, Xander Harris was sort of the show's Jimmy Olsen. While Buffy was obviously a superhero and Giles was clearly a wise mentor and Willow would eventually go on to develop awesome magical powers of her own, Xander remained the powerless support character throughout the series. The character made sense when you considered the whole gender-inversion thing of the show. With a female superhero lead, you needed a male character to fill what was once a traditionally female role. Xander was often the damsel in distress. When he wasn't getting rescued, he was comic relief and an effective stand-in for clueless viewers. With some of the best lines of the show, he also seemed to be a stand-in for the show's creator, but I'm getting way off track.
Anyway, back to gaming. I'm "currently" (if not having played in two months can be current) running a long-running game of the Buffy: The Vampire Slayer RPG. Like all successful games based on licensed properties, BtVS: RPG found a way to effectively model key structural elements of the source material. In this case, I'm specifically talking about the vast power differences between characters. Strike that. I don't think that it found an especially effective way to model power differences (that would seem easy in any game), rather it successfully made it cool to have players characters with vast power differences acting side-by-side in the same game, all with equal dramatic importance.
In our game, we of course have our Slayer, who was miles beyond any of the other initial characters in power. But the rest of the players all created what the game calls "white hat" characters. While some of those white hats would eventually go on to develop varying degrees of supernatural power (just like Willow on the show), I believe the real strength of our game is that we always had at least one white hat who can still be placed in real jeopardy. And it is our chief white hat character who has really served as the emotional core for the whole series.
Anyway, if you've read this far then I have a question for you. Are games that involve characters of vastly different level or power viable in other systems and genres? I have a suspicion that our success with this model says more about our particular gaming group then it does about any particular game or genre. I know this would have never worked with some of the groups that I games with in the distant past.