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These are categories that different fandoms, archives, and individuals tend to define differently, but this is how I prefer to define them:
-If I write a fic today where Dumbledore didn't die, it's AU. A plot point is altered, and the consequences are explored.
-If I'd written a fic before HBP where Dumbledore doesn't die during year 6, it's pre-HBP (and/or jossed). It doesn't really bother me if people say things like "this has become AU since HBP", but distinguishing it somehow from intentional AUs is important, IMO.
-If I write a fic where the gang goes to school in America, or in 1855, or where they're all reincarnated as space pirates, that's not AU, it's AR -- alternate reality. I picked up this distinction from PotC fandom, and it's quite useful.
Just including something that is extraordinarily unlikely to happen in canon does not an AU make. It's AU when you explicitly and knowingly contradict things that have actually happened in the books. If I write that Harry and Draco get married and have MPREG babies in 1998, that's not AU, no matter how ill-explained or out-of-character the fic may be, because 1998 hasn't happened in canon yet.
Personally, what I don't like are AUs written Just For The Hell Of It. JFTHOI including such reasons as "I just thought it would be cool" or "I just wish this character hadn't died because I like him" or "I just wish this pairing hadn't happened in canon because I like a different one" or "I just plain don't like this large chunk of canon".
That last one is perhaps not very common in HP, but in my last fandom (X-Files), it was rampant. People would adhere to "mixed canon", completely ignoring entire seasons of the show that they did not like, or even ignoring all episodes not written by their favorite contributors. I mean, okay, whatever floats your boat, but it doesn't float mine. Part of the fun of writing fic, for me, is using the constraints that canon provides. Doing otherwise is like saying you wrote this awesome haiku, but you didn't worry about the syllable count because it constrained your artistic vision.
Believe me, I know the pain. I did not like HBP. I did not like the shipping in HBP. It made it harder for me to write things I wanted to write. But that's part of doing fandom. It's a challenge! When I was writing For gods to menace fools, I didn't relish the thought of addressing Remus/Tonks, but I knew I had to, unless I wanted to do that thing that bugs the hell out of me in slash: pretending ships/characters you don't like don't exist.
You could maybe call this a "stealth AU". The author hasn't given it the AU label, but when you read it you find that it seems to be depicting a universe where Tonks or Ginny or Lily never existed -- or they might as well not have, since the author hasn't acknowledged their existence or importance to the characters in any way. (It's not just girls, either; James and Peter sometimes get this treatment in Remus/Sirius fic.) I expect this happens in het fic too, but since I barely read het in HP, I couldn't say.
So, what makes
It's not just that readers can tell the difference (though they can); I think authors are also cheating themselves by writing this way. Taking short-cuts in writing usually isn't worth losing what you would have gained if you'd taken the whole journey. Though acknowledging Tonks in my Remus/Harry fic took more effort, I think it made the story a lot more interesting and multi-layered. Tonks strengthens the contrast between what Remus is saying and what he is doing, which turned out to be the whole point of the story. Er, it's supposed to be, anyway.
I guess the same goes for AR, though I admit I tend to ignore those because so many of them are awful (and indeed written "just because it would be cool"). If you're writing about the gang as Americans in order to make a compelling point about them, then super!
If your fic got jossed, of course, you're off the hook. Please indicate when your fics were written, guys! A great pre-HBP fic can look like a lazily written post-HBP AU if you don't.
ETA: Ahaha, Lex and Avenger are in exactly the same pose! That was not intentional, but it is hilarious. XD