Dude. This thing has expanded way beyond what I think I can cover in an hour, or at most, an hour and a half. It prints out to three full pages, and if the panel is as interactive as I want it to be, we might cover...page one? *sobs* I may have to scale it back. Way back. Distill it down to what's actually crucial rather than what's "nice to cover." I'm pretty sure that most of these points could be covered in their own panels.
So...You Want to Write Fanfiction?
Use a Disclaimer. You're playing in someone else's sandbox, with someone else's toys. Acknowledge that. After all, you're writing fic in this 'verse because you're a fan, right? Giving credit where it's due is a polite thing to do and might keep you out of legal trouble.
Know Your Canon. Screencaps and scripts are your bestest best friends. Use them. A lot.
As an addendum to that--Know Your Characters. If you've got pre-soul Angelus giving a teddy bear to Spike, who subsequently carries it everywhere because he loves it so much, you've got a problem. And yes, that goes for post-soul Angel as well. River creeps Jayne the hell out; if you want them to get together, you'll have to deal with this issue. That being said, if you can sell the characters doing something that they wouldn't normally do, more power to you. But sell it. Don't try to shortcut characterization by saying "He started looking at her in a new way" and leaving it at that.
Research, Research, Research. Your characters and canon aren't the only things you need to know about. A few minutes on Google and Wikipedia will give you all sorts of information, and even if you don't use it, it's interesting. You want to torture a character? Fine and good, but if you want him alive afterwards, then perhaps the implements they used on heretics in the Middle Ages aren't for you. Cattle prods use less voltage than tasers. Feudal Japan has some customs you might want to know about, if you're writing in that era. Then there's Brit-picking. Elevators are "lifts;" cookies are "biscuits," the trunk of a car is the "boot;" and they don't ever say "gotten." Research helps you avoid pitfalls...and helps you avoid looking like an idiot.
Grammar and Spelling Are Important. No, really. Netspeak and Leetspeak make you look very very stupid and will get you GAFFed and FanFicRanted. Just say no to "r" and "u." You want people to read, not reach for the back button while wishing to stab you through the internet, right? Proper grammar and spelling are a way of being considerate to your readers, most of whom don't have the patience to wade through a misspelled, badly punctuated story, no matter how interesting the plot may be. People don't want to work while they read, and if they're spending more time sussing out what you're trying to convey than they are actually reading, you're not going to garner much attention. At least, not attention you actually want. Your spellcheck is a useful tool, but it's not the be-all and end-all, and it won't catch homophones. If you're weak on grammar and spelling, then:
Get a Beta Reader. Just make sure that they have a grasp on your chosen canon, or that they have other skills you can take advantage of. Also? Their word isn't gospel. You're allowed to pick and choose which suggestions you take.
Who Are Mary Sue and Gary Stu, and Why Should I Avoid Them? If your Original Female Character is eclipsing the canon characters, making them act OOC, making the boys fall all over themselves to worship her, and warping the story so it's all about her? Scale her back. Way back. If your Original Male Character is a better fighter than Methos, smarter than Giles, a stronger wizard than Gandalf, and the most handsome guy in the room? Stu alert. And be careful about turning the canon characters into Sues or Stus (Xander writers, I'm looking right at you). Good characters have flaws, and you can use them to all sorts of interesting advantage. Perfect characters = boring characters.
As an addendum to the Dreaded Mary Sue, bamfing yourself into your fandom of choice is generally a bad idea. Self-inserts are rarely well done, even when they're not as blatent as "I fall into Middle-earth and hook up with Legolas!"
That being said: Write the Sue. Embrace the Sue. Everyone does it, at least once. The first fic is like the first pancake; it's for the dogs. But if you don't write the first fic, then you can't write the second one. Get the Sue out of your system and make your favorite character happy. Then you can write the awesome stuff that you're capable of without having that hanging over your head.
Avoid Purple Prose. This: "Nighttime glistened like a fine layer of effervescent fairy dust through a shimmering layer of moonlight varnish, dripping pools of quivering mercury droplets down the crushed satin skin of Lothlorien’s fabled and powerful tree-plant sentinels" burns us, Precious. Now, it was written on purpose to be horrid, but I've seen other fic just as bad. The LotR fandom is particularly guilty of this, and purple prose often goes with writing a Mary Sue. After all, you have to describe her "sapphire orbs" and "honey-colored tresses falling to her waist in a riot of curls" somehow, and mundane prose just doesn't cut it when it comes to describing Princess Crystyl Tyffani Sylvyrmoon McSparklypoo. Work description into your narrative instead, or use simple words. "Black hair" is just as serviceable as "raven locks."
Songfic. If your fic has more song than fic, or if it doesn't make sense without the lyrics? Retool it. Using lyrics to set a tone or a mood is fine, but a character sitting around listening to Evanescence and emoting all over the place about the song is dull as dirt. Also, fanfic skates at the edge of legality as it is. Putting song lyrics in there that you didn't write? Can get you in trouble of the lawyer-ly sort. If you must write songfic where the characters are listening to music, make sure your characters are listening to music of their own era. James and Lily Potter aren't going to know who Nirvana is. Or what a CD is, for that matter. A better way to use songs is as inspiration for a story; the best "songfics" are the ones that base a plot or a character introspection around the lyrics without ever actually quoting them. Music montages are annoying enough onscreen; written out, they're just painful.
Random Humor Isn't Funny. No, really. Sugar-high crackfic with characters running into walls and screaming "Cheese!" at each other is stupid, not funny. Monty Python can get away with it. You? Not so much. Because if you can, then what are you doing writing fanfic?
The Sex Scene. Bad euphemisms only make people giggle. "Molten core" is a no-no. "Weeping cock" is just as bad. "He put his sex in her sex and they had sex"? Yes, that's an actual example, and really, you should just fade to black if you're going to write that.
POV Shifts. Pick a POV for the scene and stick with it. A reaction shot as the POV character walks out of the room can be used to good effect, but don't give your readers whiplash by putting them in three different characters' heads within three paragraphs.
Tense Shifts. Pick a tense and stick with it. Writing in present tense is incredibly hard, mainly because we don't read much fiction in present tense. Present tense can be used to good effect, but you really have to concentrate in order to do it well. A sentence like this: "It starts to get late, Sakura woke up from her daze and stroll home, unwillingly" will have readers reaching for their sporks.
Character Bashing. We all have characters we don't like. But if you have to write them Out of Character to show your extreme hatred, then you're not writing that character, you're writing a pod-person. Riley isn't a rapist. Kate isn't an emo whiner. If you must villainize a character, at least turn them into an interesting, in-character villain. Alternately, you can send them off to Chumbawumba on some errand or other for the duration of the story. Not every character has to be in every story.
Do Not Use Rape as a Plot Device to Get Your OTP Together. This is a Bad Idea. The Healing Cock does not exist in real life. This also goes with Character Bashing, because having a character you hate be the rapist? Um, no. Unless they really would do that. Most of them won't. Really.
Warn Your Readers for Potential Squicks. Everyone does not have the same taste, and what makes one person go "Ooh!" might make someone else go "Eww!" Some people would rather not read graphic sex, graphic violence, rape, mpreg, or character death, and warning for such (although this is by no means a comprehensive list) is considered polite.
Mpreg? What's that? *sigh* That, my friends, is shorthand for "male pregnancy." And besides being an abomination to biology, it's usually abominably written as well, without even a bare acknowledgment to basic anatomy. It's the sort of thing that only "works" (I use that term loosely) in a few fandoms, and even then, it needs to be thought through very thoroughly. Also, chances are a male character in any fandom who finds himself pregnant is going to freak, not start happily knitting booties. Take this into consideration.
Fangirl Japanese. Or French. Or Spanish. Or any other language. Don't. Unless your canon characters (such as "Firefly") randomly use words in other languages, don't do it in your fic--especially if you're depending on BabelFish to translate for you. If you must do this, find a native speaker, or someone who's taken classes, to make sure you're parsing it right, because otherwise the native speakers are going to come after you with pitchforks. Trust me. For Firefly, resources exist both online and offline, and they're not hard to find. Also, don't translate in-text; do it in an Author's Note at the end. In-text author's notes interrupt the flow of the story and break that wall that's supposed to be there between you and the reader.
Crossovers Can Be Fun! However, an attempt to make them plausible is always appreciated. Portals and wormholes, while overused to the point of cliche, are handy tools. If they're already part of one of your chosen fandoms, all the better. However, keep in mind that not all crossovers are compatible, and some are more difficult than others. The magical systems of Harry Potter, Buffy the Vampire Slayer, and the Dresden Files are all very different from one another, and finding a way to reconcile them makes for some fun writing. Keep timelines in mind as well. Harry Potter starts in 1991, and Harry is 11 years old in the first book. Buffy is Chosen as a Slayer at the age of 15, in 1996. Therefore, she's not going to be able to go to Hogwarts and teach a class to Harry--and neither is Willow. Dawn would actually be a few years behind Harry. You can deal with this, and maybe it seems nitpicky...but people can be rabid about details like that.
Feedback Is Important. Feed the authors. Reviews are the only payment we get. If something in a story particularly touched you, if a certain line made you laugh, pull a quote and let the author know. Constructive criticism isn't always welcome (some people are awful brats about it, in fact), but if an author says they want it, then by all means tell them what didn't work for you in the story as well as what did. By the same token, if someone leaves you less-than-flattering feedback, or you get GAFFed, receive it with grace and see if perhaps they have a point. If all you ever get is slobbering praise, how are you going to get better?