Well, I began my research a while ago now, and I think I’m ready to present my findings.
The show is fine. Every episode has at least one line that is recognizably clever, and it occasionally makes me laugh out loud.
I didn’t find it to be brilliant, though.
Basically, it’s just a cute kid’s show that doesn’t suck.
Sure, people will argue that I need to watch more, and maybe I will, but honestly, I didn’t see anything to obviously explain an enormous fandom of adults, including adult males.
…Except for one thing.
The CUTENESS.
Because, I admit, it is very, very, very, cute.
Sickeningly cute.
Like, “I can’t believe how shamelessly adorable they have made this” levels of cute, from the character design to the little life lesson that comes with every episode.
Now, I have always found My Little Pony to be cute, even back in the days when the ponies actually looked somewhat like real ponies.

But the new anime-style ponies are DISGUSTINGLY cute.
They took something that was already pretty adorable and they ramped up the cuteness by about a zillion notches.
I don’t know if they consciously followed the baby schema, but it worked.
The baby schema is a sort of list of characteristics that scream “baby” to mammals, and which is proven to trigger our instincts for parental affection. It includes characteristics like big eyes, tiny noses, large heads, small bodies, big foreheads and so on.

So, this, basically.
Teddy bears over the years have evolved to fit this schema. They went from looking like this:
…to this: 
Well, let’s go back and look at that 1980s My Little Pony again.
Big eyes, big head, short nose… pretty cute, right?
Now look at the modern version:

Next to the Friendship Is Magic version, the eighties one looks positively adult.
So, here’s what I think happened:
The most famous emergence of Bronies as we know them today came from 4chan, of all unlikely places.
A bunch of dudes from 4chan started watching My Little Pony: Friendship Is Magic with the intention of mocking the show, after getting all riled up from a post about how children’s shows are made just to hock toys (as if that’s anything new…).
Except the guys got hooked.
They started posting pictures of ponies everywhere.
It started a whole 4chan flame war, largely one sided, where people mocked the Bronies, and the Bronies just responded with pictures of adorable ponies.
Someone has even graphed the emergence of pony presence of 4chan.

And after watching the show, I think I know what happened.
The cuteness messed with their brains.
I’m telling you, after watching a few consecutive episodes, I had serious Pony Brain.
All the next day, unbidden mental images of ponies flitted through my mind on adorable little wings.
And I’m not the only one to report this phenomenon.
Sites like My Little Brony seem to acknowledge it as known fact. 
We’re all programmed to fall for cuteness. That’s how babies avoid getting hucked out the window after the third hour of consecutive midnight misery.
The excellent flash animation and the Baby Schema of the character design in MLP: FIM is like Cuteness Crack.
Our brains can’t help but get addicted the sheer adorableness.
Anyone who has fallen in love slowly with a newborn baby can understand this process. At first it’s like “yeah, ok, it’s cute,” and then you’re like, “let me take another look,” and pretty soon you’re like “my child is the cutest thing ever I can’t stop looking…”
And that’s what I found happened to me.
I started craving ponies.
And why did it happen to all of these 4chan males?
Well, honestly, how exposed is the average 30 year old man to cuteness?
Boy toys are much less cute.
GI Joe has adult features, so do Transformers. Most adult men don’t go to cute little baby movies. Most don’t even watch Disney flicks.
So imagine you take this population of people who are completely unexposed to cuteness on a daily basis and then expose them to multiple hours of this:
I think they get hooked.
It’s like exposing the Aztecs to the European diseases of the Conquistadores – they’d just fall in scads.
And I fell prey to it too. All I wanted to do was look at pictures of ponies, especially Rainbow Dash, who I like primarily because she is blue, but also because she’s not girly.

I wanted a little pet Dash to follow me around and live in my pocket. I wanted a little toy one that actually looks like the character instead of like the lifeless, personality-less plastic Hasbro crap.
In other words… I began to know what it is like to be a Brony, and to see why some of these Adorableness Addicts are willing to pay literally thousands of dollars for a toy pony.
And that explains, too, why Bronies have such a reputation for being positive and friendly, for responding to trolls with ponies instead of insults, and even for charitable donation.
Oxytocin (which is released in response to cuteness) makes us more trusting and kind to others.
So, they can justify it all they like by saying it’s the character development, or the voice acting, or the plot, or the humor. All of that is… fine. It’s not a BAD show. It’s cute. It’s occasionally funny.
But I think that the main reason that they like it is that Bronies are just addicted, just the way I am hooked on this face:

And I can tell you that I love Owl because he’s funny, and smart, and loving, and lots of other great things.
But really, I love him because of the oxytocin.
I’ve been off of My Little Pony for a week or two now, and my brain is returning to normal. The pony cravings have stopped.
But will I go back?
Yeah, probably. Even though I don’t think it’s that fantastic.
Just because it’s DARN CUTE.