I don't read canon point for point and dig for details to make sure that the way I read it is 110% canon-compliant. I rather boggle at it when people do. At the end of the day, I'm not studying it for Battle of the Books, and I don't have to know it backwards and forwards like I do the literal words on the surveys I administrate at work. I find authorial intent in the HP canon interesting in a trivia sort of way, and it can certainly color the way I read the text after that, but there's a lot of unintentional depth of character that the author has no interest in exploring the same way I do. The author is very interested in some characters who I'd really rather skim. In some cases, the author thinks she's writing a book that is different from the one I wound up reading.
One of the things that gets hammered in almost any writing class is "Show, don't tell." You can narrate that Hermione Granger is a brainy bit of a thing until you're blue in the face, but that doesn't have half the power that showing her waving her hand in the air and jumping about to answer a question, and reading rather dull books does. If narration of character traits in text is disrecommended, how much less literary credibility does it have to have an author have to answer questions about how the characters were have supposed to have been portrayed? There are things that do fit more easily into the source text than others, but some things are better left a mystery.
Star Trek spoiled me for a One True Interpretation fandom. There are so many authors and alternate universes in Star Trek that it would be foolish to attempt to insist on One True Vision, though a lot of people do. Star Trek does the alternate universe thing a lot. It's far easier to mentally label canon inconsistencies as AU rather than OMFG BLOOPER. I like some of the book-canon that got Jossed better than I like the movie-canon. When you have so much canon to choose from, and so much that should be canon that isn't, it's hard to go back to a One True Canon universe with one visionary. So much was up to the different writers and directors and actors to fill out and interpret and occasionally go beyond Gene Roddenberry's vision. A few things were even intentionally left up to the watcher. Someone posted recently about the backstory behind the change in appearance in the Klingon race, how he or she had wondered about this. Finally, it was addressed in an episode, and Worf shut down the query. "We do not speak of that." A real universe has open ends like this, where no amount of authorial interpretation can replace the sweet mystery of not ever knowing for sure.
I first encountered the concept that fanfiction could go on a wild tangent that canon would never ever approve of, and have it still be good, when I encountered the Sith Academy. There has been good fanfiction that the creator would never agree with before and since, but that was my first, and perhaps even my favorite, encounter. The Sith Academy started with the simple and creative premise that Darth Maul had to have some training before he came up against our favorite Jedi, and this was part of that training. Darth Sidious has Darth Maul performing insane and stressful tasks that are of immediate familiarity to a modern audience familiar with urban life and life on the outskirts of government and campus activities. Darth Maul's first task is to learn to drive in Coruscant's insane traffic. The characters were writ large and boldly at first, caricatures of George Lucas's vision, but as more people joined in the project and Ewan McGregor's past characters were conflated into Obi-Wan Kenobi's own past, a certain subtlety and sense of hidden anguish informed the stories. George Lucas never would have approved, especially not the idea of a Jedi loving a Sith, but the general quality of the stories and the commentary they make on the Star Wars universe are very good.
I never before thought that I could wholeheartedly believe in something as patently ridiculous as a Sith and a Jedi falling in love, but the Sith Academy showed me how someone as snarling and badhearted as Darth Maul might eventually come to care for someone (albeit in his own cranky and cruel way). I had a bulletproof OTP in the X-Files (MSR all the way!), but the Sith Academy made me realize that I wasn't looking for a particular pairing that worked when I read romantic fanfiction, I was looking for good writing, good storytelling, recognizable characterization, and artful suspension of my disbelief.
In the world of canon, many things are impossible. Darth Maul does not actually drink that brand of beer, nor does he live in an apartment immediately adjacent to that of the man who will eventually cause his hips and his ribs extreme separation anxiety. Darth Maul would not actually attempt to get in bed with Obi-Wan Kenobi. Darth Maul would snarl incoherently and attempt to seriously make diced meat out of the Jedi. But if a fanfiction writer wants to take me to a place where Sith come to grudging terms with their softer side, without giving up their core of innate cranky snarl, and Jedi re-live their misspent youth with less drugs and more responsible sex, who am I to deny their vision? All I ask is that the writer show me how things progressed from the characters we know and love to hate into the characters that they're writing.
I don't have a One True Pairing in Harry Potter fanfiction. There are too many people of nicely diverse genders who can be plausibly hooked up. I've read too many people making an implausible pairing into a very scarily plausible pairing for me to say that it can't or shouldn't be done. The only time I have trouble with a pairing is when it's written badly or there is Scary Fan Drama associated with it. I do have difficulties with Ron/Hermione, largely because it is canon and this is causing an attitude of insufferable snottiness in certain factions associated with 'shipping it. You're right, you don't have to keep rubbing it in, the author agrees with you. Big whoop. Now STFU so the rest of us can read our fic in peace. Same goes for Harry/Ginny Weasley. Canon says yay. Canon says 'pastede on yey' when I read it, actually. I do wish that JKR had gone to the same trouble of selling it to the fans as some of the fan works take to selling a more implausible 'ship. She may have sold it to some of you, but she didn't sell it to me very well. I say that if a slavish imitation of canon is what you want, you're welcome to it. More for you. For me? Give me your slash. Give me your het. Give me your gen. Give me your rarepairs. Give me your angst. Give me your comedy. Give me your crackfic. (Oh, gods, yes, the crackfic. ♥) Give me your AUs, your OCs, your OTPs. Give me your best writing. I'm in a reading mood.
What I particularly fancy is when a fan author has taken some aspect of the wizarding world that JKR is perhaps overlooking and not treating right, and expanding and exploring this in their own style. The source material has errors and omissions and authorial oversights, as do most all works. There is so much rich magical technology to explore. There are so many interesting characters. There are so many supposedly cardboard characters who must surely have their own stories. There are so many missing pieces. JKR doesn't have time to explore all of this. There are so many different interpretations of the same character, so many that sometimes you have to wonder if we were all reading the same books. I already read canon. I read the book that I was reading. It was a spiffy story about a boarding school with magic in. I could go read it again, but it would be the same book. I want to read the book that you were reading.
Just sell me the premise of the story, the premise of the changes you made, and I'm yours.
Note that the state of awake has been degrading through the process of writing this, and so, therefore, has the clarity of the written word and the integrity of the essay format.