These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Awesome Music: The main theme, especially during the finale. Also the "exit music" that plays as the crowd is leaving.
Ending Fatigue: The original Disneyland and the Tokyo DisneySea versions have a simple climax, in which Mickey draws a sword and uses it to defeat four villains, after which various Disney characters emerge to celebrate Mickey's victory. The climax of the Disney World version, by contrast, has Mickey raise fountains, then draw the sword, which he uses to defeat seven villains. Reportedly, watching all seven of them die takes so long, some viewers leave without seeing the finale.
First Installment Wins: Consensus states that Disneyland's Fantasmic! beats the other two by a long shot, although Tokyo DisneySea's version seems like a worthy contender.
Most Wonderful Sound: You've been sitting on a blanket for two hours, the sky getting darker, as more and more people gradually fill in the space around you. And then...finally...over the crowd noise, your ears barely detect a bell-like tone. Over the next several seconds it rises in volume, and all is well, because the show is finally beginning!
Nightmare Fuel: No surprise, considering how many Disney Villains invade Mickey's dreams.
Visual Effects of Awesome: Special mention for creativity must go to the clips projected against mist screens and to Disneyland's animatronic dragon. The Tokyo DisneySea version has a nifty cone-shaped structure with LEDs that form appropriate clips, and their dragon doesn't look too shabby either (especially when it appears to emerge from inside the Magic Mirror!).