Adult genre covers all subjects not appropriate for young audience: crude language, sex, violence, taboo subjects such as religion bashing, intolerance, psychopathy, etc. Sometimes it is broken into subcategories: Adult for sex/swearing/mild violence, and Mature Adult for all the disturbing concepts of psychological mind games, slavery, human trade, graphic kinky sex, drugs, torture, etc.
Adult genre offers the hardcore experience that is not available in most fandoms, as many authors dim disturbing concepts in their work in order to gather / not scare away general audience. Plunging characters into adult situations offers the opportunity to explore their psyche to their deepest, push them to difficult choices. It allows for more realistic approach, not skipping any darker stuff that happens in real life. It always impresses me when a fanfiction writer (or a regular writer, too, who is not of horror genre) touches controversial subjects. Too many people try to avoid this stuff.
In fact, many fandoms feature relationships that are rather unhealthy if we take a closer look at them. There is no better way to delve deeper into this matter than adult fanfiction. It takes character development to next level.
Adult is my favorite category - to be honest, I'm wary about general-rated stuff, as it often feels a bit incomplete to me. Though I do enjoy smut, it pains me somewhat that majority of adult fanfiction is written solely for sex. Sex, of course, is a great way to present characters in different light (and amuse the readers), but there are loads of other mature aspects that are left unexplored.
I am a rabid supporter of dark/kinky fanfiction. While it is fun to see otherwise kickass characters perform home chores and have fluffy sex, there is plenty opportunity for more. Most fandoms leave openings for dark plotlines; if you take a closer look, you will see the trauma and tragedy, twisted relationships... things you would not show to a child. Not necessarily only sex-related. Hardcore Adult combines angst, tragedy and traditional adult concepts into one.