In addition to looking plastic and cheap, one of the problems with the Little Mermaid is the fact that it's an unimaginative, almost literal re-telling of the story -- the attraction is actually framed from the perspective of Scuttle recounting the movie's events.
While Disney's best attractions typically have some semblance of a narrative (and might even be explicitly based on a pre-existing movie or story), they're effective largely because they don't stick rigidly to a script, but rely instead on certain specific aspects of the underlying narrative that invoke an emotional response and allow you to become engaged in experiencing the narrative, rather than having it told to you.
In other words: it may be a story about Ariel, but it should be your story for the duration of the ride. If you're sitting through the ride thinking, on some level, "I'm watching what happens to the Little Mermaid," then the ride has failed: it's become the Reader's Digest condensed version of the movie, stripped of all emotional content and ability to engage the guest in a visceral way.
In fact, the main reason the Little Mermaid is a disappointment is embodied in the very first "Disney keystone philosophy" that Tony Baxter included in his letter of resignation, ostensibly directed to his fellow Imagineers:
