Another episode hurt by a bad moral, but it's really fun despite that.
Let's get one thing out of the way first: Luna Eclipsed is far from a bad episode. While it suffers from the same problem as "Green Isn't Your Colour"-that is, a bad moral dominating the story-it's still a very enjoyable episode in its own right, with great humour and a wonderful new character in Princess Luna. Its to the credit of those elements that this episode rises so far above its many faults.
The centrepiece of this episode is Princess Luna, having finally made her first appearance after being cured of the dark magic that made her Nightmare Moon. Returning to Ponyville for the first time, she has issues fitting in. This character is almost immediately three-dimensional, and is easily the episode's crowning achievement. There's a fair amount of depth to her situation, and while it's not touched upon, it's there for more observant viewers. The character is also fun to watch, with her quirky demeanour and sarcastic quips.
On the other end of the spectrum is how other characters are depicted, particularly Pinkie Pie and Rainbow Dash. The two of them seem meaner than usual, with Pinkie Pie continuing to do something that's obviously upsetting another for her own enjoyment, and Rainbow Dash going maybe a little too far in her pranking. With Rainbow, it's not that bad, and not that out-of-place either, but where Pinkie's concerned it's very notable. Pinkie Pie simply acts badly in this episode, and it gets quite aggravating.
Meanwhile, the simple fact that Ponyville ponies are still telling the pre-S1 story of Nightmare Moon is problematic in its own right. It seems odd that Celestia at no point celebrated the fall of Nightmare Moon and her sister becoming good again, and for Ponyville ponies to still live in "fear" of Nightmare Moon is a failure on Celestia's part. Perhaps it was meant to seem that Luna's Royal Canterlot Voice was what initially scared Ponyville, but it sure didn't come off that way. Even Applejack for a moment judges Luna as if she's Nightmare Moon. There's no real reason why ponies should still be treating Luna like her former alter ego.
With that said, the episode is filled with great gags, and almost all of them hit. There's great humour on show here, perhaps some of the show's best, and it makes the episode very watchable even at its worst. The episode's filled with fun moments, and it remains energetic from start to finish in all the best ways. It's a very fun watch, and while Luna contributes greatly to that, she's not the only source of humour in this episode, as other characters also have their own clever quips or silly actions.
In the end, though, the moral simply isn't very good. Characters in this episode act with prejudices that demonstrably harm another, but the episode shrugs this off as them "just having fun". This seems a little harmful, because the resolution of this conflict seems to say that feeding prejudices will suddenly make people like you. That's ridiculous, and the potential of children taking that to heart is dangerous. It's not okay for people to hold prejudices. Accepting these prejudices will only validate them and make them worse. Of all the things this episode could have said, this is perhaps one of the worst.
Despite that, it's still a very enjoyable episode, and the introduction of Luna is very, very welcome. It's the weakest of the first four episodes of the season, but that's the only condition under which it's a low point. It's a fun watch and certainly makes up for its deficiencies.