I have another Pet Peeve, but not about the authors but rather the reviewers. It's been a while since I read anything on fan-fiction dot net; I quit because of the amount of pure dreck posted. But what made it worse was the number of comments saying "Oh, this is so great! I loved it! I can't wait for more!" Really? Seriously, why encourage bad writing?? I'm not saying to get all Simon Cowell on them, but rather please find a nice way of saying they need to improve.
The OC gets treated like a queen right off the bat. "I know you just graduated from Starfleet Academy with top marks, but here's a first officer position aboard the Enterprise!"
There's that one, and then there's "he's the youngest captain in Starfleet." I remember reading three stories on fan-fiction dot net; the first had "the youngest captain" at age 30 (promoted on his birthday). The next author made the captain age 29, and then the first author had a 27-year-old captain. I started to wonder how long before they put teenagers in command.
Someplace, a couple years ago, I posted the math calculating the youngest possible age for a full-bird Captain / Colonel in today's US military. As I recall, if someone hits minimum time-in-grade requirements every step, they would be 34 years old when they make Captain.
Unsurprisingly, most of these are Mary Sue characteristics.
The thing with Mary Sue is you're kind of darned if you do / darned if you don't. You write a character that's "perfect", and it's a Mary Sue. But Starfleet is supposed to be the best of the best, and besides who wants to read stories about boring people doing boring jobs?? It's a fine line.
When there's suddenly some graphic "shipping" stuff when there was absolutely no warning about that at the beginning.
That one isn't restricted to fan-fiction. Mike Shepherd put quite a bit of "spice" in the Kris Longknife series, starting about Book 5 or 6, but they were still okay for younger readers (early teens). His first Vicky Peterwald spin-off book, however, had scenes that would make many adult readers blush. Way over the top.