I used to write all this stuff in high school that was just terrible. Like, hideous purple prose, incomprehensible characterization, no action or ever actually getting to the main plot. And more recently I'd look at that stuff and wonder how I ever managed to write anything so shitty.
. . . And now I remember that it's because, oh yeah, original writing is a lot harder than fanwriting.
Logically, it shouldn't be. It should be easier; after all, you don't have to worry about whether you're conforming to a set of characterizations or a world that has already been determined by someone else. But in actuality, it seems like in fanwriting the canon functions as a sort of safety net: you don't have to explain everything to the reader, because they already know basically who these characters are and what this world is about. Write original fiction, and all of a sudden you have to decide everything: where does this take place, and when? What is the society like, how do the characters talk? And if you don't bother to decide these things, you're basically crossing the SMeyer line of laziness in writing . . . .
Furthermore, in fanfiction you can feel okay about writing things that really make no sense so long as the canon makes no sense. That's why it's fun to write for series like Naruto: you can be writing about stuff out of a medieval society one minute and then about a movie the characters went to see the next, and hey, no one can really criticize it because that's the way things are in canon. If it's original and makes no sense, on the other hand . . . .