I'm not as crazy about it as most are, but I remember an article about it.

One of the things said were that there's a lot of characterization in it, currently, Rainbow Dash, Pinkie Pie and Rarity are the ones with the most character development, the only Sue in the show is Fluttershy.

Some of the episodes are really well-written, like, for example, Hurricane Fluttershy, The Return of Harmony Part 1-2, Suited for Success, Heart's Warming Eve etc.

Some pretty cliche, like MMMystery on the Friendship Express.

Also, the for-an-episode characters are usually very original.

The songs are usually of Broadway quality, and the animation is beyond superb.

But I think that what makes the Bronies so addicted at the start(sayin' this 'cause I didn't like the pilot episodes much...) is the curiosity of what-gonna-happen-next, the animation and songs, and some interesting parts of the story.