It was interesting. I really wasn't sure about having Discord appear again initially (and I wondered if they brought him back mainly because John De Lancie's so popular), but it turned out well. I had this feeling of "unfamiliar terrain" throughout the episode, as if treading new ground; not a bad thing, though, and I feel this may be an episode that will grow on me.
MLP:FiM seems to have a thing for heel-face-turns — this is the second already, after Trixie (or the third if you include Luna). The idea behind this seems to be that noone is TRULY evil at heart; evil is learned behavior, it stems from past experiences, and it can be unlearned. Going out on a limb, I think that underlying this is also the notion that evilness is not so much a reason to condemn someone as much as a reason to pity them: it merely shows they have not moved beyond it yet in their quest for personal growth.
If that's what was intended, it's a good message, too, especially for a kid's show.
Despite this, the end was completely unexpected to me. I thought that Fluttershy was making a grave mistake with her promise, and even more so with sticking to it, yet it worked out. And I kept figuring Discord would ultimately stay a heel (and I was wondering how the conflict would ultimately be resolved), yet I was shown that even he is not beyond redemption — well played, MLP. And, yes, well played, Fluttershy; it takes a lot to pull off what you did and bet everything on one card... and win.
'course, I'm still finding it difficult to accept that he's reformed, but I presume that's because the good-vs.-evil dichotomy's so ingrained in my thinking, and also because I'm older and warier and more cynical than the show's primary target audience.
Speaking of evil, I think one could also argue that this episode underlines the point that "chaos" and "evil" are not the same thing to begin with. Take AD&D, for example; for all the flaws of its alignment system, one thing they got right was the separation of Lawful/Chaotic and Good/Evil into entirely orthogonal axes of alignment, with Chaotic Good and Lawful Evil being just as valid as Lawful Good and Chaotic Evil (not to mention that various Neutral alignments). Discord is chaotic in that sense, and I doubt THAT will change, but he needn't be evil.
I hope the show won't run out of antagonists, BTW; but on the other hand, I'm not a fan of recurring villains that never learn or change, and character development is one of the show's strengths. All the characters have developed over time, some a lot, some a little, some visibly, some less so, and I'm really, really happy with that; compared to many others shows that have a simple premise and then stick with that for their entire run, it's really refreshing.
Time-wise, this was another episode that felt like it could've benefitted from getting more than 22 minutes. The main part was very compressed, and there wasn't much time to let everything unfold naturally; the ending was also quite short, and felt liek "Discord's reformed, here's Celestia now, good job ponies, and here come the credits already".
I could feel it in the introduction/exposition, too. I was surprised that Celestia apparently didn't even inform the Mane Six of her plans; I would've expected her to tell them beforehand instead of just dropping Discord on them (quite literally). Echos of this stayed with me throughout the entire episode, too: I kept thinking "What is Celestia thinking? This can't work — and she of all ponies should know better". (But yes, see above; Celestia is wiser than I am, at least from an Equestrian point of view, and she sees much further than others.)
Speaking of Celestia, BTW, I was surprised how she appeared without any fanfare, too. I mean... just quickly dropping in on a meadow outside Ponyville? I would've expected a bit more protocol there.
Other random things?
The animated dinner table implements were too cute; they reminded me of old Disney movies (you know the ones). And Pinkie's reaction to the gravy boat was spot-on for her.
The spell that Celestia supposedly put on the Elements of Harmony seemed like a little too much of a Heisenberg compensator to me. 'course, maybe she was just bluffing there to begin with — who knows.
Also, has Dashie ever been called "Dashie" on the show before? I know I keep calling her that, but I'm not sure if it's ever been "official" before". :)