I think the thing that sort of angers people about Bronies is that they're taking something meant for little girls and warping it. It's one thing to get obsessive about something aimed at your audience. It's another to essentially steal a show not meant for you.
And that's what it's essentially is, because I've heard parents complain about how impossible it is for their daughters to search for MLP on the internet without getting some meme site or weird sex s***. That's another thing that's so much worse about MLP than other fandoms; that they've somehow taken a show about ponies meant for little girls and made a phenomenal amount of weird sex s*** related to it. It's disgusting in ways that typical weird sex s*** can't even touch.
While I haven't seen the show myself, from what I've heard of it, it's (while funny) very shallow. "Let's be friends" as a running theme is not something that any mature-minded adult should be obsessed over. It implies a level of naivety and immaturity among the fanbase that creeps a lot of people out, I think.
And this is just me personally, but the fan works I've seen of MLP have almost universally been "Take pony, stick in something else". Let's take Doctor Who and remake a scene with ponies in it, hilarious!!!!! Fan stuff of other works can get creative, but fan stuff of MLP seems to be just that, and to me that implies that the fanbase cares more about the fact that they're watching ponies than the actual show itself.
So yeah. Bronies suck, worst fanbase that's ever existed, sorry B8ers who are bronies I don't mean you.
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Black Turtle did a pretty good job.