My first overt contribution to the Harry Potter fandom was a strongly-worded rant in defense of canon in the context of fan fiction. Why is Draco snogging Harry in the Quidditch showers when on the last pages of OotP they couldn't possibly hate each other more? Why are Hermione and Snape suddenly doing the nasty in the Potions supply cupboard when last time we all checked, he was an extremely unattractive and acerbic sonofabitch and she was exchanging letters with an international Quidditch star? That kind of thing. The responses ranged from "word, sista" to "omg oppression!" and I haven't changed my tune since then. Much.
I have certainly realised that some things that may appear clear-cut in canon are not so clear-cut if you've got a different set of lenses through which you view the world -- because we all get out different things, depending on what we bring with us to the table/bed/hammock when we crack open any book, not just a Harry Potter volume. Before I started thinking about which house might fit me best, I had no particular feelings about Slytherin house and certainly did not see the hypocrisy and lack of realism inherent in attributing only negative traits to roughly one-fourth of the wizarding population (because those Slytherin kids grow up to be adults, see, most of them evil!1).
When I took an online Sorting Hat test which threw me straight into Slytherin and when I realised that the lines by the Sorting Hat that I felt the most resonance with were about Slytherin and not Gryffindor, I was disappointed. Because given canon, it's a desirable thing to be a Gryffindor -- nearly all of the positive heroes in the books hail from Gryffindor. So I felt a little at odds with the canon when I realised I could not really identify with the heroes and instead bemoaned the lack of Slytherins who represented their house qualities in a way that could be considered acceptable and even desirable. Upon a closer re-read of the books, the character who most stood out to me in OotP (HBP wasn't out at the time) was Phineas Nigellus Black. Fuck you if you think that means I haven't got my "priorities" straight. They're books. Fiction. What are these "priorities" of which you speak? Point is, we're all going to read differently.
My biggest peeve with fan fiction has always been authors' inability to stick to basic canon. Harry's eyes are green. Dan Radcliffe is not Harry; the movies are not canon. In the books, Harry's eyes are green like his late mother's. This is a canon fact which is just stupid to mess up in fanfic, unless you've got him wearing coloured contacts -- which is perfectly fine, by the way, if there's a reason for it. Draco's eyes are grey, this is also straight canon factual information. Harry was born on July 31st, year debatable (since years of the actual Trio-era timeline are never explicitly mentioned in canon; we can only infer the timeline). This is canon. Draco was born on June 5th, year presumably the same as Harry's, if Hogwarts letter-sending practices are consistent. This is canon if you consider facts JKR publishes on her website canon. I do, so to me, Harry is a Leo and Draco is a Gemini and I'm not terribly emotionally invested in making Draco a Scorpio, Cancer or Pisces (all water signs that would go well with sobbing in bathrooms, natch) or making Harry a Virgo, Capricorn or Aquarius.
There is, however, a difference between "hard canon fact" and "reasonable inference made from canon". If you can take something in canon that appears to be a certain way and turn it on its head and make it appear different, this does not make your interpretation wrong. EVER. Most people assume that Draco bought his way onto the Slytherin Quidditch team. Some go so far as to claim this is a canon fact. They are wrong to claim it a fact. The only facts we have are:
a) Draco bragging about the Nimbus 2001 brooms Lucius bought the Slytherin team
b) Hermione's accusation that Draco bought his way onto the team
c) Draco's none-too-pleased reaction to Hermione's words.
These can be read in a straightforward fashion, the way I suppose most people read it -- the brooms are in evidence, there is bragging, and Draco's not happy about being reminded that he's not good enough to join the team on his own merit. Open and shut case, folks, nothing to see here. Bullshit. The above three facts can also be interpreted in the following fashion: Draco won the position on the team fair and square, and a proud, beaming Lucius decided to reward his son's achievement. When Hermione dismissively says that Draco bought his way onto the team, this stings him because she's wrong, but he doesn't want to explain himself to a Mudblood. Just as open and shut, thanks. Not my idea, by the way; I read this interpretation first in one of sarahtales's fics (I think it was Loved Those of Great Ambition but it could have been Underwater Light). A similar procedure may be used to challenge ANY assumption you've made about canon. Your interpretation of the text is not the text itself.
Some people have really bizarre interpretations, absolutely. I don't think that there's much sane thought in the theory that Hermione and Harry's flight on Buckbeak symbolises their future eternal love for each other. I think that Snape's blatant mistreatment of Harry in class is just that -- abuse, and not hidden affection. You could make a case for it being hidden affection, but frankly, you'll only sell it to those who want to believe it in the first place -- which is what makes it and Symbolic Flight different from a theory based on logical, straightforward canon extrapolation. It is possible to get too theoretical; it is possible to stray so far from canon in your suppositions that your entire theory rests atop a pyramid of straw men who are about to go on strike. Not all canon theories are created equal; some will hold more weight than others and some will be inane. But just because you cannot see the other guy's reasoning does not mean he is automagically wrong. You might just be approaching the text from a completely different vantage point. And yes, some theories are simply batshit insane. But the simple fact that one fan's theory might not agree with yours does not make them batshit insane and you an epitome of sanity, mkay.
Just because I'm a Slytherin fan does not mean that I think all Slytherins are really swell people who are just misunderstood. I hate Snape, Lucius and Voldemort; I don't think either of them is "cool" in any way whatsoever. Snape is a greasy, unpleasant man with a ten-year-old boy's grudge and questionable loyalties; he makes Harry's life miserable and while I was willing to cut him some slack before HBP, I'm not anymore. Lucius appears to have raised a child who, albeit spoiled and bratty, possesses a functioning conscience, but other than that I have no use for Lucius; I think he's a toadying slimeball. And while I sympathise with poor Volders for apparently having been born irredeemably evil (*eyeroll*), he is a murderer and a psychopath not in theory alone; he's killed people without remorse. On the other hand, I like Slughorn and Phineas Nigellus Black. I love the Slytherin kids, especially Draco. They have the potential for growth, they can still avoid making their parents' mistakes. These are my intepretations and there are people who disagree vehemently, and that's okay, I don't begrudge them that. But neither of us is "right" or "wrong" because when it comes to Slytherin, there is too little fact and too much assumption and conjecture.
You don't have the right to claim that your way of interpreting is saner than the next guy's unless the next guy's arguments are based on pies in the sky instead of logic. There are hard canon facts. There are also matters of interpretation. Please do not mix these up, because when you do, you make my side look really stupid and that makes the baby llamas very sad. :(