 Some of them will be from backgrounds different from yours, and believe it or not, they're NOT going through the fanfic archives with a mind towards complaining about how authors screw up female/queer/poly/Black/Asian/Vulcan/etc/etc characters.  They're us, and we're all here to read the 'fic and meta the shows and have our squee.  When we write it wrong, when we stick the characters we don't have as much in common with into roles that don't fit them, when we let poor canon choices dictate our own poor uses of characters, when we use our own experiences of being outsiders from the "acceptable" mainstream (and we're all geeks here – ain't none of us who hasn't felt like the kid from Mars from time to time) as an excuse rather than a means of empathizing with our characters, well, then we fail our friends and we fail ourselves.  We fall into the trap of taking the easier, lazier path, and we teach ourselves nothing by our own prose except new synonyms for sexual organs.  It matters because when we fall to the default, when we exclude and minimize and play to norms we tell everyone, whether we intend to or not, that anyone outside of those norms does not matter, does not count as much, is not as important.  You don't want to be that fan, young fanthing.

When we take things from other cultures without making the effort to understand what those sparklies (and sometimes those horrors – again see the Krystallnacht wank) mean to the people who owned them before us, we tell those people, those fellow fans, that their experience isn't as important as ours.  That we, as tourists on a fanficcing holiday, get exactly what it's like to live every day in, for another example, a society that was established based on the imprisonment, slavery, rape and near-annihilation of one's family.  That we get to tell this story, not to bring it into the light, but to use it as a backdrop for something else, and pretend our work is more relevant this way.  That it doesn't matter anyway, because we're just having fun and people who complain are just being spoilsports.  (I am establishing a new law: Anyone who has ever had a heated argument on any board or community about who Batman could defeat [with adequate time to prepare] who then uses the phrase "It's just a show/comic/book/movie" to shut down complaints about racism, misogyny or homophobia in the source immediately loses their humanity roll and also gets a boot up the ass for good measure.)

This is hard.  It's easy to throw up one's hands in frustration and say one will never write women/minorities/gays/Vulcans again.  Given our chosen genre, that is in fact sadly very easy to accomplish.  (Except for the Vulcans.  Those guys are EVERYWHERE, omg.)