What would you say is an important thing every writer must take into account when writing fanfiction since you are basing it off a original fiction/anime/TV show/movie etc?

I also think it's a major turn off when fanfiction has OOC characters. Then, of course, you also have to take into account the fact that the author has creative license and is allowed to change any aspect of the fiction they choose and to whatever degree they want.
I believe the reason we don't like stories with major changes as much as we might is the fact that we enjoy the original so much. It was the one we fell in love with in the first place so everything after it lacks in comparison. I'm not saying fanfiction can never compare, but that the original is always held as the example of perfection.

To be honest, the important part of the character isn't necessarily having it in-character. After all, I've read so many stories that are fantastic, even though the characters aren't the same. The important part is giving them depth, knowing when to stop, and of course, keeping the fundamental core of their personality or being. It's like suddenly making Severus Snape a happy-go-lucky preppy jock, who loves pastels and flowers. It just isn't done.


( but it helps the fanfic if it's written really well, so get lots of editors! [I have 5... =_=" ]   )

I think it's important to write IC characters, but it's not necessarily the most important thing. It's great if you can mimic the writing style of the author, but I think just  basic quality and good writing is more important - at the end of the day, it's all just about messing around in the worlds you love, and there's lots of funny crack and AU fics out there that don't necessarily have IC characters - take Harry Potter and the Methods of Rationality for one! It's one of the most popular Harry Potter fanfics ever written, and there are barely any characters who act IC.

Sorry, it looks like I'm about to rant... teehee:
Since it's a fanfic, I think it's good when the cannon characters don't get overtaken by the original characters, who end up getting the spot light. It's just odd if there are so many OCs...when it gets to the point where 90% of the cast are OCs, I sort of think that the author should try a hand at writing an original story, one with their own world.
One thing I believe is important, for myself though, is that we don't... "steal" the characters. I really respect how the author has written them and how they are in cannon because mainly, it was the characters who attracted me to the story in the first place. A good plot helps too, but for me it's mainly the characters.
Anyway, so the thing is, there are times when something happens in fanfic to the character that affects their mental or physical mind so absolutely that their entire being changes from what they were before. They basically become unrecognizable. Then again, there are stories that take place over long periods of time and the characters grow to another stage of maturity and that's a hole in my argument.
I understand that it can sometimes be a plot device, but when a character gets abused by another character, whether physically or mentally, I get really uncomfortable and sometimes depending on how far it has gone, I feel horrified at what damage has been done to the character.
For me, fanfic is extending the world that has already been created by the author and giving it more shades of colour and thought. Explore the areas of the  world that have yet to be explored and so on.

This is an old topic but someone mentioned it irritates you when people write OOC ...what is the point of writing a "fanfiction" if you are going to write another episode of the same fandom. I mean seriously; if you need other chapters/episode of whatever fandom you are in...then you are not writing; fanfiction...you are copy writing. Sorry for the pun there but that is my opinion.

I have written an extremely successful series that I follow canon of the stories in the details but that is "the past". My series sets the same characters with their own futures and issue. Issue being family, etc. It has been very well received. While I try to maintain the characters from my fandoms as close as I possibly can; again...I am writing an emulation of a favorite story and continuing it as it were.