Not so much a badfic sin, but one in general. Too much 'shipfiction' for a show where the 'ship' wasn't the main focus, or where romance was barely even the focus unless it worked in tandem with the plot.

1. Soapboxing. That includes both whiny, ranty author's notes and putting your opinions into a character's mouth. Cori Falls is guilty of both, naturally.
2. Character bashing, full stop. I don't care if you don't like Aang's character and think Zutara is better, don't turn him into a whiny selfish monster just so Zuko and Katara can ditch him and go have sexytiems.
3. 'hi i dont believe in punkshuashun or right speeling and grammer also my shift keys broken'
4. "Hi, readers, welcome to the great wall of text! No silly little paragraph breaks in THIS fic!"
5. "I won't continue unless I get 10 reviews!!!"
6. "Who cares about plot and character development, it's all about my OTP!"
7. Forcing characters to act OOC for the sake of a contrived plot they wouldn't otherwise be involved in.
8. Genericization.
9. "Watch me put my favorite character through the wringer for 10+ chapters to prove how strong and brave and wonderful they are even though all they're doing is crying in a corner waiting for someone to rescue them from their trauma of the day!"
10. Die For Our Ship in any way, shape or form.
11. Rape is love, rape as backstory, sex is the cure for rape, rape as a plot device, all that bullshit.
12. Crude humor for the sake of crude humor.
13. A/N: I'm inserting my thoughts into the narrative rather than saving them for after the story's done!
14. "Here, have two pages of disclaimers and half a page of actual fic."
15. THE CLIT IS NOT A PENETRABLE ORIFICE. EVER.

Character derailment of any kind. I read fanfic because I want to read stories about my favorite characters. If they act OOC, then it kinda defeats the point. It's even worse when it's related to Die For Our Ship.

As I don't read much smut, I haven't run into Rape Is Love, but I'd stop reading if that ever showed up. I don't have an issue with Rape As Drama, so long as it's handled tactfully. One very good Buffy fic I read revolved around Willow being raped, and the Scoobies hunting down and killing the rapists.

For me it is definitely OOCness.

I can dig AUs, even everyone else is OOC but the main character who then struggles to find out why everyone behaves so strangely ( it can be done nicely).

Even a High School fanfic could be interesting, if the characters remained themselves and try to figure out what is going on. Like after that scene at the end of Matrix, Agent Smith is suddenly in a school, a teacher walks by and tells him not to be late for class. Whatever he does (like killing everyone in school, driving away etc) he always ends up back at that schoool and everyone he killed is alive again ... that might be funny if someone can write Smith very well ...

But I digress.

Bad spelling, incoherence and these things are bad awful but at least a good sign that the fanfic is only good for sporking. When these are missing you can waste much more time looking for a good story.

BTW, I tried to search here and googled - but I was not able to find anything. What is IKEA sex?

IKEA Erotica. Say goodbye to your time for the next few hours.

Thank you - do not worry I am immune to tv tropes pull. Needed 30 seconds and found my answer

Bad spelling, although sometimes it just makes it funnier.
I have to say though, if I read one more fic where a Mary-Sue goes to Hot Topic, I'm going to snap.

I understand your pain on that one. I actually had a superhero and villain destroy a Hot Topic in a fight once to work out that particular frustration.

South Park had an episode where 'real' Goths (as opposed to 'wannabe' Goths aka Twilight fans) burned down Hot Topic. It was glorious.

Incoherence and lack of commas, probably. I hate it when I can't even decipher what's going on. Rather than amusing me or annoying me, it just becomes an utter waste of my time.

For me, failure at figuring out normal basic human interactions. It is the mark of stupid beyond stupidity and makes me wish the authors experienced firsthand the events portrayed in their story.

There's a range of reactions considered 'normal' for example to being raped (going to the police; withdrawing in denial; hiring a killer; etc.) and others which are considered abnormal enough that they get dissected by the psychologist (Stockholm syndrome, etc.)
If you are going to write about the latters, it would be nice to hint that you know this is a fucked up situation, not describe how the heroine, just like that, is suddenly attracted to the dark brooding villain who forced himself upon her.

Come to think of it, has there ever been a fic where the victim falls in love with her attacker, but the rape was meant to be just that?
"Will you get away from me, woman? I fucking raped you!"
"No! You love me! We're meant for each other, you just can't admit it!"

Making strong characters into weak ones. I've heard of, though never read, fic where a character with superpowers finds out that they are being stalked by an ordinary person, and is terrified.

What I want to see is the same scenario, only the stalkee simply deals with the stalker.

That's often a product of misogyny--strong female characters turned into weak damsels for the sake of the plot. I've seen it happen to Raven and Starfire from the Teen Titans cartoon more times than I can remember.

It usually only happens to male characters in bad slash fics when they're transformed into the submissive weepy uke partner.

I love dom/sub dynamics but those fics don't exist for the pairings I like /tangent

I think the thing I hate the most is when a story has a good premise, but the author is so caught up in their own agenda that it shits all over the fic. Like The Pendulum of Choice, a Harry Potter fic which has the cool plot device of dimension hopping and a few inspired alternate universes, but messes it all up with its fixation on Harry/Snape, Draco, or Lucius slash regardless of whether it makes sense. Or Alone, Together, a Kim Possible fic with a "last two people on Earth" premise and some interesting scenes, which gets completely bogged down in bizarre high school I CAN'T BE A LESBIAN BAWWW craziness when they get back home to everyone, which the author admits is the part they most wanted to write.

Granted, both fics have other issues, but the thing that makes me tear my hair out is that the author's pet topic crowded out all the good ideas they had.

In my opinion, fanfiction could benefit from some more lesbians. Maybe we can slash Catherine and Nelly Dean.

...What?

Grammatical bullshit annoys me far more than the average badfic sin. Nothing makes me hit the back button faster than a wall of text with multiple quotes on the same paragraph, or sentence structure that failed fourth grade.