I cannot remember having seen this as a child; but watching this the first time I was really moved. The episode focuses on depression and how society "To sad to play dodge-ball? That's ridiculous, now let's see some enthusiasm!" and especially parents handle it (both are helpless and while Homer treats her as a child, Marge tries to force her to happiness and sees Lisa's unhappiness as her failure as a mother). It is the first episode after five rather Bart- and Homer-centric episodes that focuses on Lisa, and the first episode that gives her character some real depth and also gives it directions for the rest of the series (after she has been shown as slightly as brattish as Bart in the previous two episodes).
I especially loved Homer in this episode, who even though helpless, behaves warm and fatherly towards Lisa. And then of course there is the music. While I am not a fan of Jazz and the Blues as such, I really love how the saxophone music is integrated into the episode. The tunes and lyrics are catchy. It's also the first time (besides in the Intro) that we see Lisa play the saxophone in an episode.
I feel like this episode addresses some real problems, combined with humor, critical commentary on society, good music and a great conclusion of the conflict that Marge and Lisa have, there is once more also some critic on the school system ("I hope we don't see any unbridled creativity again"), and I can even relate to it on a personal level. And on the negative side? Well, I cannot find anything, this time. For me, this is one of the must sees if you watch the Simpsons.
The sixth episode, "Moaning Lisa," is about how Lisa gets depressed about problems around her and she meets a jazz man, Bleeding Murphy. Meanwhile, Homer and Bart engage in a video game wrestling competition that usually has Homer losing. But, Homer may be out for revenge....
+First appearances of: Ralph Wiggum, Jacqueline Bouvier (Marge's Mother), Mr. Largo, Bleeding Gums Murphy.+The idea for it was suggested by James L. Brooks, who wanted to do an episode where Lisa was sad but she did not know why. The writers also felt that they had done several "jokey" episodes on the show and wanted to try something new that was "really emotional and sweet".+Bleeding Gums Murphy and Lisa are playing their saxophones on the same bridge that Homer tried to kill himself on in The Simpsons: Homer's Odyssey (1990).+Bart's sentence: I will not instigate revolution.+The title comes from the painting entitled "Mona Lisa" or "La Gioconda" by Leonardo da Vinci executed in the 16th century which now is residing in the Louvre in Paris.+The song Lisa sings in this episode later reappeared in expanded form on The Simpsons Sing the Blues CD.+Sofa gag: Maggie pops out, Marge catches her.+Bart's phone prank to Moe's Tavern: Jacques Strap.+The original idea for this episode came from James L. Brooks who wanted to do this episode clear back in the days of Taxi (1978). The idea was that Alex would be sad for some unknown reason, but the script never came to be.+Some of the video games that can be seen at the arcade: Time Waster, Freeway, Pac-Rat, Eat My Shorts, Nuclear Disaster, Robert Goulet Destroyer, Escape from Grandma's House, Itchy vs. Scratchy.+Lisa brushes with Glum Toothpaste.+The designs of the boxers in the video game Homer and Bart play were loosely based on Homer and Bart and the referee in the game was based on a character from Matt Groening's Life in Hell comic strip.