I agree with all the points given!
Personally, I don't really consider visual/ kinetic novels as a traditional game. While some do have fail states, most are interactive fiction with branching paths. In this regard, more choices and even more importantly, remembering these choices by having consequences to the story could make the novel better. If a visual novel is what you're aiming for then the standard features (save, quick save, etc.) are more than enough since the main driving force of a visual novel is the story itself. The addition of minigames and others could detract from the experience if they were just added for the sake of it.
In terms of developing an adult game, in my opinion, you should first decide on what is you want to focus on. Is it a porn with game or a game with porn and then work from there. Either you have sex front and center with minor gameplay elements or develop a fun gameplay loop with sex as reward and of course a story or something to give everything context.
From what I've seen, in this forum at least, is that there are two types of players of these games. The first is the one who enjoys the game and gradual grind for the happy reward and those who just want a quick and dirty fun time. Games usually target both these types by well, being a proper game for the first type and then offering a gallery unlock code for the second type.
Anyway, what I appreciate in an adult game is the context. Well if you think about it, this is just an expansion of the eroticism that was mentioned. It adds a lot to the sex if there is proper build up as to why are you banging this chick or something. The events leading up to it, or in the case of a VN, how you are able to affect the situation in order to lead to this banging. What I really want to say is, I want to bang characters, not just walking sex dolls.