The epilogue does jar me in two significant ways, and in one I have a reader's eyes, and in the other I have a fanfiction writer's.
1. Harry/Ginny is dreadful.
2. Bitch chewed up nineteen years of my fanon.
Does any of that need much explanation? The romance of Harry and Ginny sucks. Harry feels nothing whatsoever for Ginny for five books--any moments of interaction they have are devoid of any emotional engagement on Harry's part--and then we are, violation of all storytelling violations, told that Harry suddenly feels passionately for Ginny. I don't give a shit if that's how romance happens in real life. Storytelling doesn't have shit to do with "how it happens in real life." Storytelling is about engaging the reader as the narrative is spun. Storytelling is about making it happen and making the reader feel as the protagonist feels without trying to tell the reader how to feel. Harry/Ginny does none of these. Harry/Ginny is shitty romance, and the epilogue throws that in my face yet again. (Wow, that's three uncensored "shit"s in one paragraph. Don't hold back, Amy, tell us how you really feel.)
And it's the fanfiction theorist--I include fanartists in that, because their work creates fanfiction in graphic form--who gets upset over the shutting-off of nineteen years of open canon. The "whatever you want to imagine it to be from this point on," cut short. Nineteen years. That entire stretch of narrative boxed in by that epilogue. Restricted. Caged! How dare she!
I mean, I would like to whine that the epilogue is all so unabashedly heterosexual, isn't it, but that's just an extension of "Give me back those nineteen years so that I can correct that crappy romance and add all the other stuff I like to imagine instead." Can't fight that scene. Can write around it if you want--can get damn clever in writing around it--but it's canon. Epilogue What Epilogue has become its own dominant AU subspecies, and no wonder. Epilogue no fun. Epilogue go 'way.
So I think the fanfiction kind of fan is more likely to dislike the epilogue than the casual fan, for reason that it makes our sandbox tinier. Though I think both kinds can have the taste to hate the Harry/Ginny for just being blah.