Well, I'm not sure what happened with my first upload, but I'll do it without any formatting shenanigans this time.
Please please please PLEASE for the love of God use spellcheck. It takes literally a minute or less and there's literally no reason not to. So many fics have very basic spelling errors, even ignoring homophone errors I've seen. Please, just use spellcheck if you aren't gonna at least try to not make spelling errors as you go. It's not hard and it'll increase the quality of the story THAT much.
If you have to explain a joke, chances are you need to rewrite it. Or you are just underestimating your readers' intelligence. Yes, I understand that Ewe2 is the Zootopia equivalent of U2, I don't need you to explain that in parentheses literally right after. Chances are your reference isn't that obscure that you need to explain it. And if it IS obscure enough that you need to explain it, then change that shit. Most readers don't care about how obscure your music tastes are. They are there for the story.
Speaking of music, please for the love of all that's good, don't put the lyrics of songs in your story. It kills the pace and is almost always completely unnecessary. It's almost always just filler. And if by chance the song IS relevant to the situation somehow, whether as a parallel to the situation or because the lyrics make the character think of something, or whatever, please just stick to the relevant passage. We don't need to see the whole Goddamn song.
Speaking of pace killers, don't put Out of Story inserts into the middle of your chapter. It's jarring as hell and unnecessary. ESPECIALLY don't do it to make some sort of joke. Chances are whatever joke or reference you are planning to put in isn't nearly as funny in practice as it sounds in your head. If you really REALLY have to explain something, put a footnote in and explain it at the end. No need to hold up the story for everyone who doesn't need the explanation.
This is really more of a personal pet peeve than anything, but I like it when fics stick to what's established in the source material. I mean AUs are one thing, I like AUs, but when a story is otherwise supposed to be in the same canon as the original, it's nice when writers try to actually stay within the constraints the movie sets up. So stuff like humans in Zootopia, especially without any magical explained reasons for them being there, name changes, using "gods" instead of "god" singular without any explanation of why (they established in the movie that they have the same religions as us basically, if they hadn't this wouldn't bother me, but they did so it does. Maybe I got autism or something.), etc. Like I said, this is more of a personal issue than an objective one. And fanfiction is your oyster, you can do you, but I know what I like so I figured I'd throw that in as a last bulletin point. But feel free to ignore this one.
Don't ignore the other points though. I think they are self-evident.
That's all I got off the top of my head,