My Little Pony: Friendship is Magic - Image #305,328

I actually have a theory.

Fluttershy clearly has rage issues (which we have known about since the end of season 1), and I suspect that her normal emotional state is the result of a serious case of repressed emotion. This is not to say that she would normally be mean, but that she would normally be about average due to her kindness being balanced out by her anger, leaving her basically normal. When Discord cursed her, the repression turned in the opposite direction. Now instead of being a basically good normal turned saintly, she was a basically normal pony turned obnoxious. Iron Will, however, did not reverse Fluttershy's emotional makeup but instead simply removed the repression, which caused the anger to overflow unchecked. Thus, while Discord's method left behind some latent kindness which rendered Fluttershy more annoying that dangerous, Iron Will rendered her purely and totally angry without any check. This also explains why Discord Fluttershy did not act the same as she did at the Gala – the effect of having the animals abandon her there was basically the same as Iron Will's speech.

TLDR: Fluttershy is Equestria's Bruce Banner.