There are a large number of tropes that pop up in every genre of art, but porn is so seldom shaken up that tropes and cliches abound. Which is not necessarily a bad thing. The speed with which the story gets to the sex is appreciated by many, and is only really possible by the use of shortcuts. The list below will be in no particular order, and will be largely ignorant of the ebb and flow of tropes on porn over the years since I am not a historian.
Common hentai structure: In my experience most hentai has a flashy sex scene first, most of the plot, another sex scene, and then at most a page of concluding plot