I have an actual issue and a pet peeve.
The peeve is inconsistency with tense. I'll write in past or present tense (though IMO past tense is better for narrative), but for the love of all that is sacred pick one. And don't go against the one the game is already in, damnit.
The actual issue is people saying they're up for something "dark" and then being suddenly taken aback when my character does something terrible (but in-character, like a mercenary assassin killing an innocent witness) or engages with mature themes like abuse and/or trauma. If you don't want that, sure, I won't force anything on you, but don't TELL me I can bring my asshole murder hobo in need of second chances to grow away from her past if you aren't actually willing to go there.