Aaron, I'm going to ramble as it comes to me, but I've thought it over and I still can't nail down the exact reason why I find adult MLP fandom unsettling. And while I don't share the same concerns as screw or Anf (the predatory grooming line of thought), I believe our thoughts stem from a similar social perspective. I watched the Brony documentary, I've read brony threads on dedicated MLP forums, I've watched brony youtube videos, and common denominators for the majority of these adult males are: low confidence, immaturity, and poor social skills. My concern is that the Brony movement is encouraging and enabling what I consider shitty human traits. This, for me, is what sets apart bronies from many of the other types of fanatacism you listed. From the study I cited it was something like 90% of heterosexual brony males were single and roughly 60% wanted to be in a relationship. Guess what's more than likely stopping them? Low confidence. Immaturity. Poor social skills. The very characteristics that are being reinforced by brony-ism.
And you know, for kids like Angus- that's fine I suppose. He's young so I guess that's a valid excuse to watch My Little Pony. But men my age? They're celebrating a show that teaches surface level deep lessons about the magic of friendship. Yeah, it's a great thing to reinforce with children 2-11. But it's bewildering that a 20 or 30 year old man actually takes anything away from these little life lessons? It just isn't admirable to me. I guess that's my issue with it. Their obsession with little, colorful female ponies is socially harmful to themselves.