Damn near perfect.
There's very little wrong with "Hurricane Fluttershy". This is one of the most delightful episodes of not only the season, but the entire show. The script is dripping with cheese, but it hits the bullseye that makes the cheesiness wonderful. The dialogue is excellently written, dripping with emotion in almost every line. All of the drama and sentimentality congeals into one glorious package.
Fluttershy's characterization here is fantastic. She's afflicted with some sort of anxiety disorder, and while I can't attest to its realism, as a character trait it's a brilliant challenge to overcome. It's a great character trait that provides for a very satisfying hurdle to see jumped over. Meanwhile, her relationship with Rainbow Dash is adorable, though at times it seems almost too close. Rainbow Dash's characterization, meanwhile, reflects Rainbow's values well, and her attempts to understand Fluttershy are a continuation of her increasing signs of maturity.
Tonally, this episode stands apart from other episodes from almost being overbearingly dramatic. Somehow, the elements all come together to hit just the right spot where it enhances the episode instead of harms it, with intense music merging with the dramatic dialogue and emotive animation. Dialogue is excessively dramatic, yet highly enjoyable because of it, with plenty of cheese to be found. Meanwhile, the animation uses plenty of subtle touches to convey heavy emotion, which strongly complements the musical score. The effect is more than a little cheesy, but because of that a delight to behold.
While animation benefits from the crisp high definition and bright, colourful character models and backgrounds, not to mention the aforementioned emotive touches, motions themselves are at times a bit iffy. There's a fair number of moments where a pony's limb twists or contorts in a way that it really shouldn't, and while this is rarely distracting, it's occasionally noticeable. Meanwhile, Fluttershy's anxiety comes to a head in a short moment, well known in the fandom, which is awfully frightening for this show.
"Hurricane Fluttershy" is a positively brilliant episode, filled with drama and emotion, as well as strong characterization and signs of major character development. It has a personality all its own without losing the spirit of the show, and every element comes together in a perfect little package. It's a personal favourite of mine.
P.S. I bet FlutterDash shippers got a lot out of this one.