You know, I would usually go for nice plots and plots that involve some choice for the player in game, but then I remember every time I make 100 save files and savescum to get the ending I want and I get over it.
To me, considering everything, what is most lacking in adult games is variety. Some adult games do have some variety in both the kinks they touch and the scenes they do, but cept for those VERY few exceptions, I literally find myself skipping over 90% of games cause they're just same scenes, same "my parents died"(Some people call that story, I don't), with different characters, over and over.(And sometimes not even different characters)
Going a bit more into the "systems" category, I'd say a compelling RPG leveling system, where I can actually have fun with the combat and not just smash the strongest attack(Or just normal attacks) over and over. I also don't like games that stall me so I can only level up in a limited fashion to keep me under control. If you don't want me to powerlevel to oneshot, give me interesting stuff and skills along with more limited stats, instead of only stats.
And finally, I've yet to find another adult game(Besides one I'm gonna mention at the end of this) that mixes a story I want to actually follow and porn scenes properly, to the point where if I want to fap, I'll play this game, no matter if it's got a story to follow, cause it doesn't make me go zzz when I want to(like I said) fap, and at the same time I'll be thinking about what'll happen to the characters when I'm finished playing.
^^^^ In this case I'm talking about Tails of Azeroth(Whorecraft), I followed the story, fapped to the scenes, and kept being interested in the story enough to keep going after that. And that's what I think most games miss, mixing a good story with porn in a non boring way.