I think I would have to say research and details. I also consider this my greatest weakness as well since I often tend to add to many details sometimes as well.
*gives you a hi-five and seekrit hand-shake*. Yes! I know exactly what you mean. But one good thing about this is that you learn a lot when you're writing a fic--granted, sometimes a lot of it doesn't end up in the fic, but it was fun.....
You are not going to screw up--you're going to write a gem, and I can't wait to read it!!