I love the underdog. The way I see it, if someone is going to save the world and win the girl, he damn well better have to fight hard for it. The victory is sweeter that way. Besides girls, answer honestly, do you REALLY want the guy who has had everything handed to him on a silver platter, or do you want the guy who ahem, APPRECIATES, what he's got?
From the Disney-fied King Arthur, to the Goonies, even the Frightners (I know there all movie refs, but even a movie started out as a story), the Zeros turned Heroes give us the internal warm fuzzy. They make us all feel like we too can accomplish anything. Common, look at Bill Gates!
In a story, the Z(h)ero is the perfect plot device. Sometimes you can see it a mile away that the 14 year old kid with a crush will someday become a big bad werewolf and save the day, and sometimes you are blindsided by the one kid in all of Hogwarts that never did anything right saving the day for Griffendor.
But like all plot devices, if you use it too liberally, it looses it's power. Did you REALLY have to put the Z(h)ero so obviously right there at that moment? We all know that the 90 pound weakling will become the 250lb muscle man in the end. He probably wears blue tights and flys too. Be creative with it.
Once again, I have to confess that I've got nothing accomplished on my book in... well months now. Still haven't finished my timeline. I have nothing more on my Z(h)ero than a name and that he has a penchant for human blood. I think I'm going to have to abandon my "writing on a whim" scheme (that's been working sooooooooo well ;)) and just sit down each day and type. Blah