In fact, the only 'wrong answers' for this or any other Darker Side, are to either a) attack or demean the use of or appreciation for a topic in fiction, or b) assert or imply that using or appreciating a topic makes a person more mature/intelligent/aware that "it's just a story" than someone who avoids it. I don't anticipate either of these with this community, but the internet is a very 'once burned twice shy' place, ya knows?
[Warning, discussion of incest, both underaged and non.]
I chose incest for our first topic, because it is a very broad one and can allow some meta-talk or branching off to more detailed discussions, and because it covers a wide range of 'darkness'. I'm going to throw out a few rough distinctions in describing a situation as 'incestuous,' and criticizing them is as welcome as using them to shorthand your posts.
Underage : Obviously, when one or more party is a child. A dark topic on its own, it is very often a factor in the darkest of incest related fics.
Imbalanced : Imbalanced is my short hand for a situation where one party holds a clearly superior familial relationship, such as a parent or guardian, grandparent, Aunt/Uncle or much older sibling or cousin. Age (both comparative and age of consent) can play a role, but one party does not have to be underage for an imbalanced situation, and both parties being underage can make it more balanced.
Non-Genetic : While incest is usually thought of in terms of a biological relationship, adoption, step parents/siblings and other factors can lead to a plot with no genetic relationship, but still being categorized by most as incest.
Non-Familial : On the other end of the spectrum, both fiction and real life give us many examples of close genetic relatives who are unaware of their connection feeling a strong attraction to each other. This is called Genetic Sexual Attraction (though the Hubby has asked me to harness the power of the internet to get it widely known as 'the Skywalker effect' so feel free to tag that way! *grin*) and while it still carries strong taboos, it is possibly the most common 'not really dark' use of incest in fic.
So, chat, write, (respectfully) argue!