One of the few things that annoys me is when they get all subcultury with vocabulary - inserting 'pony' in every word possible, turning 'everybody' into 'everypony', etc. I don't recall seeing it at all in this forum (I don't doubt it happens in the megathread, but that doesn't matter to me)
That and 'pride.' I haven't seen it a lot from bronies, but I've seen it from furries, and it's stupid. It makes sense for gays, but these things don't impact their lives anywhere near as much as being gay.
Nothing else (that I can think of) annoys me. I don't even care if someone likes MLP research. As long as I don't see it, it doesn't affect me, and I don't care.