It's just basically a good quality show all around. Good animation, good writing, good voice acting, etc. Usually you wouldn't expect a show meant to sell pony toys to little girls would have any further value to it, but they decided to make it good anyways.
Funny thing is, I didn't detect very much adult humor. Maybe a couple of subtle implied references to adult content that goes over the kids' heads, but other than that it's almost 100% kids content... which makes it fairly unique for a kids show with a large adult following. I guess it's more like they set up situations which make sense for a kid, but have further deeper meaning when you think them through from an adult perspective. Like the episode where Rarity is designing the Grand Galloping Gala costumes. Totally rings true for anyone who works in an industry where the client has artistic input. (as a web programmer, I totally identified with Rarity there)
Okay bad stuff now:
A couple of episodes seemed kinda preachy to me. Like the episode with the Zebra. Yeah, racism is bad, I get it. And wow, these are some really freakin' racist ponies.
And the episode with Pinky Pie's future-predicting power. Yeah, religious tolerance, I get it. And being an atheist myself, I don't really like the anti-skeptic message of that episode.
And they didn't even try to hide the cowboys and indians allegory in the episode with the Buffalos. Once again, freakin' racist ponies.
Also... Spike. Is it just me or does Spike represent everything wrong with Equestria's class structure? He was literally enslaved from birth. And though he's been developing friendship with the ponies, you can see from their interactions with other adult dragons that the moment Spike matures they're going to cast him out like yesterday's garbage. Freakin' racist ponies.
Oh my god, I think way too much about this damn show.
Confound these ponies!