Because of his insanity, whenever Batty Koda 'switches channels' he's could be hallucinating in some form. The 'acid' parts of the "Batty Rap" could be interpreted as him seeing things, hearing voices in his head and speaking to himself. On the other hand, the wires and antenna could very well send radio signals to his brain, and his emotions and thought process are affected by the "channel" he's on. (If it's a cheery channel he's in a good mood, if it's a consumerist channel he acts like a sales person, if it's a patriotic channel he's ready for war, etc.)
Crowning Moment of Heartwarming: After Zak admits that he was helping with the destruction of the rainforest which turns all the fairies against him, Batty - who spent the majority of the film hating humans - seems genuinely touched by Zak's remorse and tells him that he's "Not half bad for a hominid".
Crowning Music of Awesome: The movie's score, especially when the spirits of the trees are summoned and when the rainforest is restored by the seed of light.
Draco in Leather Pants: How many fans feel about Hexxus. This is what happens when you make the villain more likable and entertaining than the rest of the cast (and have him voiced by Tim Curry). So much so that it's resulted in an accidental Broken Aesop; Hexxus is so much cooler than all the other characters he actually makes you root against the protagonists, and Toxic Love makes pollution seem so awesome it's probably responsible for spawning an entire generation of future Captain Planet villains.
Ear Worm: Pretty much every song in the movie. Toxic Love stands out because of the above trope.
A Dream Worth Keeping is a far more unpleasant, though fairly common earworm from the first movie.
From the sequel, "Here in FernGully".
Ensemble Darkhorse: Pips has a large number of fanart dedicated to him- impressive for barely 10 minutes of film. This can be chalked up to his clear affection for Crista (bonus points for seemingly not being returned, or not nearly as strongly), his rivalry with Zak, playing the pan flute, and subtle but strong character arc.
Evil Is Cool: Hexxus is one of, if not the most memorable aspect of the film thanks to his Nightmare Fuel-inducing design and a terrifyingly charismatic performance by Tim Curry. Many viewers have joked that the film's Anvilicious environment-protection message fell flat mainly because Hexxus was so damn entertaining that they wanted him to win against the bland and overly-sweet protagonists.
Ham and Cheese: Tim Curry and Robin Williams were clearly both having a blast playing their respective characters Hexxus and Batty.
Misaimed Fandom: Hexxus has a huge following. Just read the comments for "Toxic Love" on YouTube. But this is what happens when you let Tim Curry do speaking and singing parts for your villain character; he just might become an Anti-Role Model.
Narm: The kangaroo mother's Skyward Scream in the second movie, where she drops to her knees while surrounded by flames and shouts "MY BAAAAAAABYYYYYYYY!!" The overly dramatic moment comes out of nowhere, and just looks silly with a cartoon kangaroo.
Narm Charm: For some, it still manages to be pretty sad.
Rooting for the Empire: Many watchers sympathize with Hexxus, who for the record is the incarnation of pollution in a heavily Anvilicious cartoon about how life is precious and pollution evil. He's an abomination whose end game is to kill all life, but that's what you get for casting Tim Curry as your main villain and utterly forgettable main characters otherwise (save for comic relief Batty).
Sequelitis: Take a baseline of Ferngully, then add the general degradation afforded to your average direct-to-video sequel, then remove the two best things about the first movie (a.k.a the voicework of Tim Curry and Robin Williams), add some horrendously cheesy musical numbers, and you have a case of sequelitis with only one sequel.
But it gets better! Sideline the heroine of the first movie for borderline cameo appearances; and for the new main character, you take a fan favorite - albeit one with little screentime or influence on the plot - and remove the character traits that endeared them to the fanbase in the first place to make for an idiot to drive an Idiot Plot. Where can you go wrong with that?
Some Anvils Need to Be Dropped: Okay, so the movie was a little poorly conceived on the whole... but Batty and his rap are generally considered to be a legitimately good message because, in the decade prior to the film's release, a lot of things portrayed in the rap was done to animals in testing. (Look up the Mary Kay Corporation's history of animal testing sometime - or don't, if you have a weak stomach).
The rap was censored in the theatrical cut because some of it was just too gruesome for a G rating, and it still squicked people out. In the uncensored song you can clearly hear the fairies gasping in horror as Batty tells them what humans have done to him, and his own screams in the background, most likely as memories.
FernGully tends to get some credit for inspiring some of the animal rights legislation later in the decade (having the bat voiced by Robin Williams undoubtedly helped).