Some of its approaches are flawed, but it's too much of a riot not to love.
"Keep Calm and Flutter On" is one of the more divisive points of the third season. It's not as commonly disliked as "Spike at Your Service", but there are common points that most people take issue with. While I agree with those, the episode still stands as one of the funniest the series has to offer.
This episode features the return of Discord, and that lives up to all of its promise. Discord continues to be a huge jerk, if relatively docile. This results in a constant stream of hilariousness, with one funny moment after another. I was for certain laughing almost constantly while watching. The return of Discord can't be taken as anything other than a very, very good thing, as it allows for an endless well of comedy to draw from.
That's the episode's biggest strength, and it's something that really overpowers the problems the episode has. This episode isn't perfect, and its biggest issue is well-known. At the end, Discord's reformation almost appears to make him docile. That would remove what makes him enjoyable as a character, but thankfully this misstep was ignored in S4. Still, Fluttershy's methods of getting Discord to realize friendship are a bit manipulative, and while I can see what they were going for, I don't think the way they went about it was the best.
The episode also boasts some splendid visuals. The re-appearance of the Elements of Harmony after two seasons without them looks better than ever, and the animation is pleasant all-around. A bunch is happening on-screen at any given time, and it always looks good. Visuals are inventive, and animations are smooth and often complex. It's a good-looking episode.
Two more left to watch, then we're at S4. "Keep Calm and Flutter On" is one of the best episodes of the season, with great comedy and nice animation. In some ways it missteps, but that doesn't detract from how much of a blast it is to watch.