First, the succinct version (since I know you've been missing NoFace; I was trying not to use him so often because I knew I'd be needing him for this one):
Oh man. Oh, gee, we're at THAT episode. And right after yesterday's episode too. Give me a minute to compose myself, guys. Okay.
One of the things that I have heard tends to alarm people when they revisit "Fraggle Rock" as an adult was the fact that this children's show with colorful Muppet creatures who sing and dance never shied away from the subject of death. As a matter of fact, death is all over the place in "Fraggle Rock". The Rock could be pretty terrifying; there are tunnels that can spirit you away to another world, bottomless pits, predatory caves, and vicious monsters lurking around the bend. Simply put, the show wasn't afraid to get dark as f**k if it needed to.
Yet somehow, "Gone But Not Forgotten" tends to get mentioned most frequently as one of the saddest episodes (the other saddest episodes ever being "Marooned" and "River of Life", naturally). And I think the reason for this is because while this is a story of how Wembly befriends a strange creature who passes away very suddenly, it doesn't really focus on that character's death. It's really about Wembly's grieving process.
That's more unusual than you may think. A lot of children's media approaches the subject of death, but most of them shy away from the subject of grief, because it is so much more painful to deal with. (Hell, I'm having a hard time writing this review!) But really, the sudden empty hole in your life is the very hardest part of suffering a profound loss. It hurts so much, especially when it's a more personal loss, as is the case here. Given that Wembly is, in many ways, still the most innocent character in the show, it's a tough thing to watch him suffer so, let me tell you.
None of the other Fraggles knew Wembly's friend Mudwell, and the episode is wise enough to show how awkward and upsetting Wembly's personal grief is from their point of view. For my money, this is the most devastating aspect of this episode. They all want to help, they hate seeing their friend in such a state, but they just don't know what to say.
Rewatching this episode, it's poignancy reminded me a great deal of "Luck of the Fryrish" or "I Remember You". And it made me wonder if the writers of "Futurama" and "Adventure Time" were influenced by "Fraggle Rock" in a way that runs deeper than the obvious "very funny series set in a richly imagined fantastical world populated with weird but endearing characters". They at least share the same mindset.
In the end, Wembly discovers a way to keep Mudwell's memory alive, thanks to a very strange quirk of his friend's life-cycle. And it involves song, which is a lovely touch, and a very Fraggly thing to do. And the song's lyrics include some beautiful images that foreshadow the events of the final episode, which is just around the corner...