Besides what has already been said, there's one thing that is a big problem within fan-fiction writing. It is a rarity for it to appear, but giant walls of texts without any space between can definitely scare readers away, me among them I have to admit. Another thing that is problematic is when entire chapters or fics only have sentences in them, not even a little bit of description in them.
Ah, the text wall. That's a terrible problem I admit. One of the basic rules of writing is that every time someone new speaks, they get their own paragraph. That includes action too. Giant text walls are almost unreadable and usually turn me away from stories unless they sound REALLY interesting, and even then I have to space them out in word. It's a courtesy to both reader and writer. For the writer, it makes things easier to edit once you finish. For the reader, it makes it easier to read, especially for the likes of me since I spend a lot of time on the computer so I regularly have some level of eye strain. Actually, I wrote a story once as a text wall, and looking back on it, my first thoughts are "how did I even read this?"