We all have them.

These are mine.

• Poor characterization of a canon character. I cringe when people play a canon character and it's clear in their writing that they don't know what they're doing. It makes it hard to want to write with them.
• OCs that are designed to be direct blood relatives of a canon character (unless it's been discussed with that writer, beforehand). Siblings and children, especially, because oftentimes you're just kind of expected to accept them.
• OCs and canon characters that directly defy the canon rules of their source material. Of course, there's always headcanons. However, this is coming from personal experience:

  If we're in a world where everybody can use elemental magic and everybody specializes in one of five (air, water, earth, fire, spirit), don't go adding in electricity as a sixth (when it's not even an element) and say "this is totally okay!".

  In a world that has strict rules down about what can and cannot be done, it's a bit Mary Sue/Gary Stu to come out and say "my character can do this" when there's not a single mention of it in canon.

  Expanding on your character's abilities in the element of air? That's perfectly fine. Those things aren't so clearly defined, so you could make up some stuff just as long as it fits.

• Metagaming. This is when you take OOC knowledge and apply it to your character, IC. The only time I ignore this is if the character I'm writing with is a mind-reader. If my character doesn't know that your character can read minds, they're going to continue their subconscious thoughts as they were. If your mind-reading character takes what my character is thinking, and responds to it verbally, that's okay. However, what isn't okay is when your completely ordinary human character with absolutely no special powers whatsover takes one my character's subconscious thoughts (not even their body language), and responds to that in whatever way, that isn't okay.

  Example: My character and your character on a flight together and they're in the same row of seats, maybe even next to each other. They've never met. My character accidentally forgot their pillow at home, so now they're frustrated. They keep thinking about how much they'd like to have a pillow, but they're also shifting in their seat, trying to get comfy and use their arm somehow.

  Metagaming: Your character, who's never met mine and doesn't know them, gives my character a pillow and tells them to "shut up about it". How did you know I was thinking about that pillow? Also, rude.

  Not metagaming: Your character notices how my character keeps shifting around. They can see my character is visibly uncomfortable. They remember they have a spare pillow in their carry-on, because they like to sleep with extra, and they point out my character's shifting. They offer their spare pillow to be nice.

• This is one of the most annoying things to me. It's when other writers do not treat my characters with the appropriate amount of caution. If my character has reputation of being a certain way, there needs to be some element of OOC respect, there. I don't care if your character disrespects mine IC, especially if that's how they are, but you should still show in your writing that you acknowledge my character could seriously mess yours up, and that you're ready and willing to accept the consequences when shit hits the fan. I can tell when somebody is being disrespectful OOC, and it gives me even more joy to reprimand their characters IC, especially when I've given them more than a fair warning OOC.
• I don't like writing fight scenes, but sometimes it's a necessity. Even confrontational scenes can be a bit iffy for me. But there are appropriate ways to respond, and then there are inappropriate ways. This kind of goes with the above but it's not fun to write confrontations or fights with somebody that's just... entirely apathetic about what's being done to them. If you get slammed up against the wall, grunt! If you're being held up by your throat, sputter, gasp for air, grab my character's wrist, kick your feet. Anything to show that you acknowledge what's being done. I tend to ignore people that aren't the least bit reactionary because they think it makes them look "badass". No. You don't look badass. You're annoying. Goodbye.

  Furthermore, if you are in a fight, give somebody at least two chances to dodge your attack before you decide to land a hit. If you give them the opportunity to dodge every time, and they take the dodge every time, that can be considered god-modding.

  Not even the best fighters get out of a battle completely unscathed and it's boring to write with Graceful Garys that just never get hit or hurt, because they're so graceful.

  On the other hand, don't go straight for the kill, either. Maybe slash their arm with your knife. Shoot them in the thigh with whatever weapon. Scratch them and get their skin lodged up under your nails.

  There should always be some balance of give and take in a good fight scene. Make us think for a second your character might not make it out of this, alive, and then push through to show "hey, I just got the hell beat out of me but I want to win way more than you do, so I'm coming out on top".