I think writing quality. If the game is only an hour back to front with an amazing story that's a win. If it takes 30 hours and the story sucks then its still slop despite the quantity
well, it shouldn't bee too long or too small. it always depends on the person reading it.. for me, well i like the middle xD i like a good read, but not too long
For me it would have to be pacing. If you don’t have pacing from say a fun mini game to good dialogue to an arousing sensual buildup than I quickly lose focus and interest in a game. The game Mist does this perfectly in my opinion
A quality AVN need to have a good story, but sometimes there can exist bad stories or not necessary bad just non existent with hot scenes because of build up or the kink they explore, or just for the context of the scene. (For example Inn My Wife very short RPGM, or Holidays with daddy very short VN)
Thats why i think it will depends what are you looking for, there is bad stories with good build up and kinks that you may like that dont need to be great VNs, just have enough good writing to deliver.