Is that it's not canon. Now, I know that people have general feelings about how things would go that aren't covered in the text (movie, TV show, book, etc) and other people have fully formed opinions on details. But I do think that when you're creating something for an audience, it's useful to let them know what you're actually *changing* from what we know (Cas is deferential) or things you're assuming that the show hasn't stated (the Doctor "danced" with Rose while she was a companion).
But there's stuff that makes me jump up and down—can you imagine someone writing a SPN story with Victor in it and not knowing he's black? Like—why would you pick something to write about that you've clearly never seen the canon for?
My current SPN ire is people who don't realise that the most popular picture of Dean's handprint is him looking in the mirror. It's on his left shoulder, people! Way to pull me out of your moment...
When it comes to properties you like, what things that people assume or carelessly change bother you?