Even though i think it depends, gun to my head i would say Graphics.
Other people have already said it here that graphics/art are good for attracting the attention since its something that unlike gameplay or plot you can judge at first sight.
But the third option which i think trumps all and...
I'm no expert in VNs or anything but here's what i think that may be useful to you and OP: Visual novels often attract the type of players that want a good immersive story, so lack of sex scenes earlier on should not be a problem at all, however i think it's beneficial to have some teasing...
I remember it being like a fast food experience, good amount of scenes but gets repetitive like other games of this kind. Won't disappoint but also won't have any surprises (i played it more than a year ago though)
I am gonna say characters but probably even more the immersion you can create with a good set of personalities. Maybe that's the main point but the game has to be solid in the other aspects as well