I just tried three times to put it into good words, and I'm still failing.
Ultimately though it comes down to a genuine love for the thing in question.
If a person loves it and wants more of it, then they're a 'true fan'.
If a person likes it and might check out more but its no big deal, they're a 'superficial fan', or a casual fan like >>4166
But the 'problem' group, I guess, are the ones who have barely checked out (or not at all) the thing in question, but act
like the people who love it.
Those are fake fans.
Stuff like cosplaying.
That's the kind of thing that you usually only get from the big time nerds who love the thing to death.
So then someone who's never even watched it comes along and cosplays the protagonist beucase they just like the design - that rubs a lot of people the wrong way.
I personally dont care so much about that - but I think it more comes down to 'cosplays the protagonist becuase the show is super popular at the moment' more than anything. And that's kinda where the issue originally grew from - people using fandoms as a cheap popularity boost, without actually giving a damn about the fandom or the thing its a fan of.