LabRat, I don't think that it is a problem with writing original characters, so much as writing original characters in the context of fan fiction. The problem there is that there are a couple of unwritten... not rules... exactly, but conventions / dangers of which I think the writer needs to be aware.
1) there is the danger that they might turn into Mary Sues, as Hazel said.
2) there is the danger that they become the focus of the story. Okay, so using original characters works sometimes, but I think there is a danger that they tip the balance away from Lois and Clark (or other series' characters), who, after all who the story is supposedly about. (But remember, there are exceptions to every rule; Irene's Firestorm etc series works very well indeed.)
So long as original characters stay in the background and advance Lois and Clark's plot they are fine.
What makes original characters intimidating, however, is that you have to flesh them out. With established characters, you can jump straight in to the story. We know what Clark looks like. We know what his motivations are. Ditto Lois. The challenge then is to get the established characters to react to the situation in which you place them.
Original characters are different. You need to sketch enough background to show where they are coming from, but not so much as to be boring. That is a quite different skill. The challenge with established characters is to keep them in character. With original characters, it is to give them a character to begin with, and make it one that the readers can react to and sympathise with.
I want to write original fiction, too. Oddly, I find that less daunting than writing original characters in the context of fanfic. There I can do what I like; they can react off each other. They can take over the whole story. That doesn't matter. It does here.
Does that make any kind of sense at all?
Chris
