I think the show’s biggest problem was structure. It needed to give itself more room to expand, and to breathe in general. I think there are two key ways they could have done this, and they were starting to do it in the later seasons.
The problem with having a moral each week is that morals are final; you’ve learned a lesson, now you’re never allowed to make that mistake again. Any stories that could have arisen from similar mistakes are lost forever, unless you want to be accused of character regression. It puts the characters on a shrinking platform. Not every episode has to end in a character learning something. Sometimes the fun is the journey.
If you want a really compelling character, give them an issue that a person could spend a lifetime wrestling with. IT can be a source of conflict, and it can be explored from multiple angles. That’s what made Fluttershy, Twilight and Starlight work, and also Luna, Discord and Tempest, to a lesser extent.
The other thing the show needed was repeatable concepts. When I think back to another show I love, SpongeBob SquarePants, I can think of a dozen episode archetypes. Who SpongeBob was paired with and where the episode was set completely changed what kind of episode it was. There were episodes about The Krusty Krab, jellyfishing, boating school, Patrick, Squidward, Sandy, Mr. Krab, Plankton, Gary, Mermaid Man and Barnacle Boy, and more. Friendship is Magic got better at this later on. There were Luna episodes, Maud episodes, Daring Do episodes, CMC episodes, etcetra, but those were the exception rather than the rule, and I feel like the show lacked a unifying structure. I would have loved “Rarity goes on an adventure for an outfit materiel” premise, and the different characters that accompanied her would give the episode a different flavour. That’s a big part of why Season Eight was my favourite: the School gave the season some framework to hang episodes off of.
@Tirimsil
I thought the episode did a really good job of portraying depression; all the things that normally make a person happy just stop working and everything feels grey. In that context, just telling Pinkie to just “get over” the instrument would send an extremely bad message. Unfortunately there aren’t any easy answers when it comes to depression, so the ending doesn’t really work. I think they may have bitten off more than they could chew.