Alef:
however, that my preferences have a tendency to sneak in the backdoor; I have certain ideas of what "belongs" in a spanking story and what does not,
Yes, but we have so many "graders" that these individual preferences tend to average out. Also, our preferences aren't a static thing. They tend to be a moving target. We may "mature" as readers and gradually discover new genres that we hadn't tried before, and those new experiences may gradually or even suddenly inform our preferences.
Also, we all have little "flings" as readers and so have temporary preferences.
Alef:
I am not very keen on explicit descriptions of genitalia
Just as we have our "likes", we have our dislikes; and they matter! Again, these can be a moving target. I developed one particular new dislike while reading through the challenge stories, but recognized it was probably just me. Therefore I tried not to let it impact my grading, though I believe I did comment on it. There was some "reverse sexism" going on that started to grate on me. Males were depicted as impulsive childish creatures who, at the slightest sexual inspiration, would drop their pants to play with themselves, even if there was a significant chance of discovery. Females hardly ever acted that way.