It's like the Tim Tebow phenomenon, people resent him because he's "in your face" with his religion, despite the fact that by all accounts he's genuinely a really, really good dude. How fucked up is that. You see, the problem isn't with Tim Tebow, the resentment comes from the insecurities of the people who hate him. I submit that most (very deliberate choice of the word most, i.e., not _all_) people who really have a hair across their ass for bronies, do so out of resentment. They resent that someone else has something that they're really into, resent that someone is part of a sub-culture that they know nothing about, resent that someone has this special thing that makes them happy.
On the part of the bronies, they probably have a tendency to be overly enthusiastic as a defense mechanism for liking a "little girls" show. But that's really kind of irrelevant. Whether or not they're 'in your face with it', this is the internet, don't read their posts.