Well, in slice-of-sex doujins, the female will normally be incredibly upfront on the topic of sexuality. The male will be often times reluctant or even refuse to engage in any kind of perversions. This is usually in order to appeal to the doujin's main demographic of largely shy males. Eventually, the main characters will have sex.
In the wouldn't-it-be-hot-if-this-character-got-raped type doujins, the female lead will be accosted by one or more males. After a brief confrontation the female would fall prey to gang. After the opening vaginal penetration an occurrence of either anal or oral rape will ensue. After double penetration is achieved, the third piece of the puzzle is added. Note that the initial anal penetration will often be associated with sharp imagery, and a very tense pose on the part of the female. The doujin is usually ended by a hearty bukkake scene. Knives usually play part in this kind of doujin.
There are near constants in doujins. For example: the woman will cry, stockings will stay on, and men and women always climax simultaneously.