It's just good writing. The characters have depth and likability to them...moreover, the show is wonderful escapism to the misery that is every-day happenings. One could immerse themselves, at any age, into this world where race, sex, and bigotry are basically irrelevant. It's peaceful, colorful, and overall positive and happy.
It's a fantasy world.
I hated Twilight Sparkle until it became evident that she was the brainy one, with incredibly competent abilities, and overall a level-head. She has her flaws, but she fucking saved the world from eternal darkness...why does everybody on the show forget that? She also got rid of a giant monster bear. Never acknowledged again. wtf...
I didn't like Pinkie Pie at all until I got used to her quirkiness. Her going insane when people were ignoring her was awesome too, but I'm generally annoyed by her songs.
I liked Applejack from the beginning. Same with Rainbow Dash, and I don't care if she's a dyke or not.
I didn't like Fluttershy at first until I realized she's just an innocent ultra-hippy. With Aquaman-like powers in communicating with animals through sheer charm.
I hated Rarity....absolutely loathed her.... until the episode where she was making all her friends dresses and they were never satisfied. Then I completely related to her, because the client NEVER knows what they really want. That's why they hire the creative talent. Even professor's warned in the industry they'll commonly just say "make it cool" and the talent is left befuddled...just like Rarity when Rainbow Dash was like "needs to be 20% more cooler." You can't work with that kind of direction but it happens ALL THE TIME and the client has to be right.
Spike needs to grow up, Luna dropped off the fucking map after the pilot, Celestia needs more depth, the great and powerful Trixie needs to become a villain, that Gryphon needs to return, there needs to be more, stronger male characters, the cutie mark crusaders are somehow adorable, and I have no shame in admitting it's a damn good show. Can't wait for season 2.