ARG. Editing is important, I know, but it drives me crazy. And I have a lot of trouble letting mistakes stay in my story once I realize they are mistakes. So I can't seem to put the editing aside long enough to completely finish the first draft... though is it really a first draft considering Chapter One has been edited about 10 times?
That's what tends to happen to me with more original fic though. I worry so much more about doing a good job on it that I get writer's block or I fiddle with the same sentence for an hour to make it perfect or I rewrite entire sections just because I didn't like the dialogue in the last version. With fanfic, I guess I feel like the bar is set so much lower that if I make a few mistakes, people will forgive me as long as I've written a good story, which lets me churn out the words without upsetting my internal editor.
It also helps that I rarely change anything in a chapter once I've posted it! Posting seems to stop the editing monster dead in its tracks. =P