Let's recap Discord's crimes: "He turned Ponyville into the chaos capital of the world!" "He made us all act the opposite of our true natures!" "He made it rain chocolate with not even a dollop of whipped cream!"
They get the first deus ex machina out of the way pretty quick... then discord eats the second one. Wait -- why does a small town public library in equestria have twenty copies of a 'reforming' spell anyway? What does it doooooo? I guess we'll never know.
The ending was, uh, abrupt. Not that he does a heel face turn with no foreshadowing; that's kind of reasonable. It's that Discord listens to reason once, so they declare him reformed and roll the credits. OTOH, it doesn't look like anypony actually trusts him much.
...but why exactly *does* Celestia need his magic, anyway? We'd better find out before the season ends!
Anyway, good episode (lots of adorable chaos, not as stupidly random as the last time), but the ending left me a little cold.