I dabble in fanfiction from time to time, whenever a story insists on being written or I just need a bit of a creative outlet, I suppose. But even when I'm not writing it myself, I enjoy reading it. It seems like even though you're using characters already conceived, the possibilities are limitless and people amaze me with their own ideas and creativity.
One thing that drives me absolutely crazy is when people insist on using Japanese in an English fiction. Japanese is a beautiful language and I understand anime is Japanese, but I don't seen the importance of adding words like "onna" or "nani" or "hai" to an English fic. It simply looks off and even to someone who speaks the language fluently (or nearly fluently anyway,) it causes a break in the flow of the story for the second or two it takes your brain to switch from English to Japanese to English again. I suppose to sum it up, I believe when writing a story, stick to a single language unless absolutely necessary. It doesn't enhance the story in any way.