Hey Eckker,
Welcome to the club. Anyway to answer your question, for me it is a blend of the story and of the art… but mostly the story.
Don’t get me wrong, the story needs some good artwork as well. If you don’t like the art… you might not care to hear the story. Case and point, Peach Girl has a good story, but I don’t care for the art much, so I quit reading it.
But when you blend a good story with great art, then you have a full winner. It doesn’t need sound or voices or anything. Tenkyusei (I think that’s how it’s transliterated) “Transfer Student” from JAST doesn’t have voice at all, but the story and art are very compelling. For a second example, Little Sister Kana also doesn’t have voice support, but has such a gripping story, you don’t really care.
Yes, the voice adds something to the game, but, to me, it isn’t needed. Criminy, I can remember playing those old Zork, Leisure Suit Larry, and Kings Quest games. Those used the fun that is a good story and a good adventure game to give you an enjoyable time. So for me, it is the story. Bishoujo stories tend to be considerably more mature (in thematic elements, nature, and whatnot), and well written.
Oh, as an aside, there are a handful of adult English bishoujo games. Sierra used to offer the Leisure Suit Larry games, which were tounge-in-cheeck bishoujo comedies. And Aftermath Media offers “Tender Loving Care”, which is a very erotic, mystery, thriller.
And there is one mainstream game that I would consider bishoujo, and that’s the X-box game "DOA: Extreame Beach Volleyball."
Anyway, just some quick thoughts.
Mike