Inconsistencies in stories, nothing throws me off worse than this. I can handle the odd inconsistency here or there, or even a completely random out of third field moment. I played a game a while back and it still rubs me raw now. The MC was in a coma for several months after a car crash which killed his father. When he was released from the hospital, no one came to pick him up. The only one who really visited him was his younger sister.
When he returns home, his mother was like "Oh, you're here, I bought you a cell phone, off to work now bye!" and she asks you to wake up your older sister who is sleeping nude. She finds out you tried to wake her up nude, and walks in on you jerking it in the bathroom only to blackmail you. Bitch, your father just died, and he was in a coma, chill.
Another problem I have is the slutty character archetype. While I don't believe every character has to be the sweet, innocent virgin type, but this very type is often thrown in with the slutty best friend, family member, whatever, and instead of allowing the relationships/characters to grow in a meaningful way, here comes some character to force the story along, making everything that lead up to that moment feel rather irrelevant. There are other examples of this happening without the use of the slutty character, but it feels like this is the go-to answer for many devs.
Lastly, I know it's a porn game. Yet, not everything needs to be sexual. I don't need a sex scene or peep scene every five minutes. Yes, it's a porn game, and most of your audience want the porn. Yet, let's face it, are you popping a boner every single time a protagonist peeps on his mom in the shower? A lot of the time I find myself just trying to click away these kinds of things. I want the porn, yet I don't need it at the cost of the game/story.