i posted somewhere a while ago a decent explanation... might have been in skype dont remember
honetly though its a bit hard to explain, the easiest way to to check it out yourself, if you watch some episodes try to stay for the first 5 before dropping, the first two you wont have appreciation for till after you finish the season
also you can check out this site:
http://www.equestriadaily.com/it has all the awesome news and fan work that shows up on the internet
the basics i guess though are these:
the art is amazing, its vector awesomeness with the best flash animation i have ever seen
the story is well done, the characters are actually developed and have depth, though episodic the story carries over into the next episode
hasbro doesnt give two *censored*s if your make something using their stuff, its free advertising to them, so unlike the legal letters you get from things like harry potter about your fan art, you actually get positive feed back from the people who work on the show themselves
the people that work on the show actually have a two way relationship with their audience, they pay attention to us and show that in the episodes, commercials, and interviews they produce
the community of fans is amazing as well, they are constantly producing high quality work and always have a surprisingly happy demeanor. they conquered 4chan and are after the rest of the internet and theyre doing this without a single mean spirited word