For me, first and foremost is the artwork. Although I am not good enough, I sketch in a manga style so I need to reference others' artwork in order to improve my own (this also applies to normal manga and anime for me). As a result, I save a lot of images onto my computer for future reference.
Although it's pretty irrelevant with Hentai doujins, the story comes second. Studio Kimigabuchi's RETAKE series proves that doujins can be great in their own right without all the sex. Everything else is next.
To sum it up: for me, Art + Story = :D