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.