Looking back at the episode, I quite like how most of them were turned by the... bad types of their element.
Applejack - Brutal, painful honesty
Pinkie - Mocking laughter
Dash - Being loyal can require betraying someone else
Rarity - She just gave into greed
Fluttershy - You can't twist kindness into a bad thing
Except Fluttershy's, none of the elements are perfect virtues. Discord's making them abandon their element by showing them the darker sides of them.
/awesome