I like reading fanfic. For all the usual reasons; for more stories about characters I've already been introduced to, for stories that never got to happen in the original, and for a certain amount of snark and deconstruction.
But also as an exercise in writing. I am fascinated in watching how all writers, no matter what the media, take on the challenges of telling the story. Setting up the situations, setting the moods, explaining the back-story, establishing the characters, the whole lot.
And this is why there is one habit in fan fiction I find really annoying. Sure, these are established characters, and an established world. You aren't faced with having to get the audience up to speed with who is who and what they are up to before that audience gets bored waiting for something to happen and wanders off in search of some other story.
Except you should still have to do it.
Here's a little tidbit from a product of the pulp era:
The journalist took a full breath and began to spread enlightenment.
"Listen, old chap, this bronze man is known as one of the greatest surgeons. As a chemist, he has made discoveries that your children will some day read about. The bronze man is rated a wizard in the field of electricity. Furthermore, he - "
The thin man in the coveralls put a bony finger against the scribe's chest. "How many blokes are you tellin' me about?
"One."The above occurs a few pages into "The Thousand-Headed Man," and is introducing, of course, Doc Savage, the Man of Bronze, the leading protagonist whose name occurs on the cover of the magazine. Except the above is from the 17th Doc Savage story, only one of what was over 181 stories in all.
And every single one of them introduced Doc to the reader.
Other serials did the same, though not always with Lester Dent's cleverness (the thin man had donned said coveralls as a disguise in a previous scene, and in fact already knew full well who Doc was, having come to gain his aid.) The Hardy Boys, for instance, made do with a drab recap of who Frank and Joe were in terse and omniscient narrative voice.
Contrast this to fan fiction. In the vast majority, all that you will see of any character is, "Harry was there, and Luna." In a smaller number (and all of the better written ones), there will be a few signifiers here and there; Luna will mention her father and The Quibbler, show off a few bits of her peculiar lunacy, and even possibly get a "...her blond hair" here or there. Extra points if none of these are either plot-important, or the pro-forma Stations of the Cross.
(To explain the above; there are things that are expected by the fandom. Jack O'Neill is going to have to say "For crying out loud" at least once, someone is going to have to mention Buffy Summers dying -- she got better -- and no possible Picard will ever get through a day without his Earl Grey, Hot.)
Point being, few writers attempt explaining who each of these people are in the same way they would have to if those characters were original. Sometimes they will do this for settings and situation, but this is more likely to be navigational aids towards when in the series this particular book is picking up. Or as part of the background of truly new or original characters; the description of Hogwarts, then, is not meant for us, it is meant to inform us of just what this outsider character knows as of that moment.
So, sure, there's reason to dislike this on purely practical grounds. Not all readers will have read all the sources, especially for a massive multi-universe cross-over. Not all readers will remember all the minor character from Harry Potter (there are a lot of characters in Harry Potter), and not all readers will figure out whether you mean movie version, book version, or some version that only shows up in the computer game.
But it is also a loss to the story, and to the writer. For the latter, you are skipping a chance to learn and practice how to slip in all of those details that describe a character. For the former, each story is a unique take, and implicitly will end up with a unique portrayal. By leaving off any attempt to describe anyone, you take away the best lens for seeing this characters afresh and making them your own.