Pet Peeves (subject to growth):

Readers, please do not:
Offer corrections in the comment section. You are NOT an editor, and your unsolicited corrections not only contribute nothing to any discussion of the story, but are almost always incomplete, and are occasionally incorrect. I've seen a number of comments that think they caught all the mistakes in a chapter, but I caught anywhere from three to ten times as many. Please understand, I know the temptation to give corrections; you genuinely enjoy the story, and you think you are doing the author a service by helping their work to be free of error. Here's the thing. Taking your corrections won't free the chapter of error, and unless you have a massive amount of tact, correction comments come across as uncaring spellchecks without any meaningful discussion/critique of the story as a whole... not to mention they come across as a slap in the face to the editor(s) who pored over the chapter for hours, eradicating most of the errors, while also thinking of ways to improve the story.
Start insult-fests or flame-wars in the comment section. I understand this is the internet and such things are often unavoidable, but don't be the one to propagate it. It is entirely possible to have a civil discussion over a disagreement without resorting to name-calling or profanity (not to mention the first one to do so loses by default). I've had plenty of long discussions about politics or religion that remained cordial on both sides, and both people involved learned something about the opposing side. If there must be a discussion, keep it civil; if you must start insulting, take it to a private chat. Besides that, there is very little to be gained from any such exchange unless it is definitively pertinent to the story being commented on.

Writers, your turn:
"You" is for subject and object; "Your" is possessive; "You're" is a contraction of "you" and "are". Get it right: "You're an idiot if you can't get your pronouns right." Why is such a small gripe first on my list? Because it's EVERYWHERE!
"It's" is a contraction of "it" and "is"; "Its" is a third person neuter possessive. Get it right: "It's sad when a story's punctuation is its downfall."
When "well" is used as an interjection ("well, I think..."), you must have a comma after the "well".
Learn about different types of dashes (hyphen "-", en dash "–", em dash "—") and when to use them. Hyphen is fairly versatile, en dash is used to separate time or distance, em dash is used like parenthesis or for cutting off dialogue abruptly. There are more uses for all of these, but those are the most common for writing fiction.
Point of View (POV). If your writing involves more than one character's POV, try to keep them separated by chapter or section. If you are particularly skilled, you can show multiple POVs even within the same section, but never put more than one POV in a single paragraph. The more POVs you have close together, the more difficult it is to keep characters' thoughts apart; having multiple POVs in one paragraph makes that near impossible.
Inconsistent quote marks and apostrophes. Fimfic does not automatically fix basic ‘single’ and “double” quote marks like most word processors do, so if you write the chapter in a word processor but make changes in Fimfic, there will be a mix of “modified” and "unmodified" marks. Whether you use modified or unmodified marks, please be consistent.
Spelling/grammar in general. This is the age of computers and word processors. You do not need a hired team of proofreaders to spell things correctly (an editor should be there to help you pick out and resolve plot holes and retain continuity, not help with spelling). If you can't afford Microsoft Word, then use Google Docs, Apple Pages, or even Grammarly. Any of these will also check for some grammatical issues. There is no reason for the massive amount of misspelled and ungrammatical tripe I see in so many stories, many of which are top trending on the front page.
Inconsistent spelling of words with multiple spellings. I'll grant you, some words can be spelled more than one way (e.g. "grey/gray" or "griffon/griffin/gryphon"), but whichever spelling you pick, be consistent with it.
Poor use of font effects. Just because bold, underline, colors, superscript, bullet points, etc. exist, doesn't mean you should use them. Italics is for emphasis on one or two words, a character's inner thoughts, and certain other effects. Bold is not for extra emphasis, or for emphasizing italicized thoughts, but for headings or certain other rare occasions. To emphasize a word in a character's thoughts, just de-italicize it.

That is all for now, but the list is subject to grow as I remember more things I despise in writing.
Notice what is not on here: profanity in stories, dumb ideas, and poor writing. While I don't like profanity, it is up to the author whether to include it. And while dumb things and poor writing are not likely to be read by me, they are a natural part of practicing authorship; everyone starts somewhere, and I encourage all aspiring authors to keep practicing to get better.