When the grind and/or exposition/dialogue to sex ratio is ridiculous. If I wanted to click through 50 screens of dialogue and exposition I'd just watch a visual novel. If I wanted a long grind before any meaningful payoff I'd play Hearts of Iron or Stellaris.
Little glitchy things like using bad renders(part of clothing clipped out, e.g.).
Lack of creativity or innovation when it comes to using a stock scenario("high school/college age MC at home with "landlady" and 1-3 "roommates", goes to college/high school with hot classmates and teachers, has to achieve some random tasks including working a job, etc., takes approximately 3 months of game time before actually fucking anyone(so that dev can milk patrons as long as possible)"