Borderline Content
The following things are not strictly against the rules, but they have a much higher chance of being deleted than normal. Uploaders should be careful about uploading these things, and approvers should be careful about approving them.

Extreme fetish content. This includes extreme guro, scat, bestiality, vore, "hyper" fetishism (monstrously large body parts), diaper fetishism, and other extreme or unusual fetishes.
Explicit furry content.
3D porn or SFM porn.
Hard translated comics.
Cosplay photos.
-[Self uploads.]
Screencaps.
Third-party edits.
Duplicates.
Bad anatomy.