Okay so after reading a lot of yaoi (both great ones and repulsive ones) I come to wonder about this question. Basically I think that Yaoi/Shounen Ai depict idealistic stories that satisfy people's fantasies and their desire to temporarily escape this mundane world lol. However, I also get the impression that hot scenes, interesting/bizarre themes, or even beautiful art styles themselves can't single-handedly make an attractive yaoi that just lures people to continue to read them (or a great story) -- regardless of whether they are sad or fluffy, predictable or unpredictable, etc.
(this question sort of takes off from my recent experience of re-reading love or hate, and no offense but I just got really bored at the plot and wonder how did the story end up here regardless of having all those exciting elements -- photographer/model, love between brothers, childhood love, beautiful art style, love triangle, etc.
Feel free to provide examples so that I could also get some yaoi recommendations haha! ლ(´ڡ`ლ)