Facial expressions are key, at least for me. And not only in the sexy scenes.
Then again, altough I like it, the art style is not the most impostant thing in GGGB imho. And I'm not alone in that regard, I think, the last poll in the patreon page was precisely about that ("What makes GGGB stand out?" was the question) and the winner option was "Choice mechanics and player agency" (424 votes), then "Writing and storyline" (345 votes) and being a "Female main character" (330 votes) almost tied, and last "Art style" (227 votes). Sounds about right.