I do actually write a fair bit of original fic - I just don't let anyone else read it! (Actually, I think you might have read some of my original - maybe I'm thinking of someone else though.) I feel like it needs more polish than the fanfic.

Mostly, I use fanfic as a way to hone my craft. For example, I was playing with tense in the first chapter of Death - trying to do it all in present tense in an attempt to create a feeling of immediacy and panic. BTW, how well did that work? The other major Buffy story I'm writing all in first person, from the persepective of an outside character (post-Chosen, so a new slayer, who's also and old slayer.... intrigued yet?)