Fanfiction.net pet peeves:"Update soon!" or "Why haven't you updated yet?! UPDATE" or "Sequel!" in reviews. On the first two points, hello, writers have lives that don't revolve day in and day out around writing fanfics. It'll get done when it gets done. Plus, forcing out writing to meet a deadline can result in a crappy end product. On the "sequel" point: Oneshot means oneshot, sequels don't always work, and sometimes I've seen people leave reviews for stories requesting sequels where one wouldn't make sense. Can it.
Pairing portmanteaus. Combining two names to make a "cute" coupling name just looks stupid. Unless you're Richandamy from Zits, it's just going to come off as cloying.
Fanfics whose descriptions always feel a need to include a list of all the couples in the story. Thanks for killing any suspense! Can't have that. Especially if it's done as Name/Name or Name x Name, or if it turns into a daisy chain or some demented love decahedron from Hell where everyone's involved with and cheating on everyone.
The word "Drabble". I hate that word. Just call it a short story. Or just a "short". "Drabble" sounds... kind of gross, actually.
Many misspellings in the summaries (not to mention the stories). Come on. Although ESL situations are an exception, since that's definitely understandable, but nonetheless: Beta readers are your friends if you can't do the job yourself for whatever reason.
Author notes included in the body of the story. They're annoying, pointless, and break up the flow. We don't care. Put them before the fic starts or after the fic ends. They're extremely distracting.
"Random" or "insanity" used in a summary. When what you're writing bears no relation to the fandom you're writing in except for names, or when the characters aren't even shadows or husks of who they normally are, but different people entirely with the same names and looks... quite frankly, it looks terrible.