The Little Mermaid is a very different story from Tarzan though. Tarzan's story is a bit nebulous and the way they did it in the theater made it even more so. The Little Mermaid is more focused--there's a mermaid, she's in love with a guy, she does what she must to get him with a few obstacles along the way, she wins the guy in the end.
What makes the movie is all the stuff they put on it--the singing fish, pretty songs, fabulous set pieces... is anybody really that concerned about capturing Ariel's psyche appropriately on stage? I begrudge it a bit but I think Disney could make a massive misstep and this would still run for half a decade.
yr ronin,
joey