1. As much player agency as possible. No ero-content happens to MC without asking you first, or warning you that a route contains it. If you want to be a total prude throughout the entire game, you can be. Oh, and MC either starts out inexperienced or you have that option on character creation.
2. Lots of options for ero-content, but not much of it dependent on different, unrelated content. I've played VNs that have overall "Slut" meters or numbers, with a lot of scenes having a number requirement. It's not the worst, but it forces you to do content you may not care for, just to drive the number up for other scenes you do want. I'd rather have a really good writer, and different variations for each scene depending on MC's past experiences, rather than being locked out of them entirely. Though I realize that's not always feasible.
3. Non-con is a big no-no, but I love to see surrendered consent, where a submissive MC willingly gives up the opportunity to stop whatever happens to them next.
4. Tomboy protagonist. I'm not attracted to masculine bodies, but excessive femininity (in appearance or mannerisms) puts me off and is a downside if I'm supposed to relate to the MC. When given the option (usually in text-based games), I go with an androgynous character that has female genitalia.